The Navugabo Groundsel was described in 1986, the species is known only from the type specimen that was collected in 1935 (according the the IUCN) or 1947 from a swampy area at Lake Navugabo in the Masaka District, southwestern Uganda.
The species was never recorded again despite it originating from a otherwise fairly-well collected area.
This species was described in 1933, originally as a variety of the Subcordate Melanthera (Wollastonia subcordata (A. Gray) Orchard) from the island of Hawai’i.
The Poplar-leaved Melanthera is known only from the type material that was collected in 1918 somewhere in the Maunalei Valley on the island of Lana’i in the Hawaiian Islands, it is now considered extinct. 
 Warren L Wagner; Harold Robinson: Lipocaheta and Melanthera (Asteraceae: Heliantheae subtribe Ecliptinae): establishing their natural limits and a synopsis. Brittonia 53(4): 539-561. 2001
The African Vernonella inhabited a quite small area of coastal grassland along the eastern coast of KwaZulu-Natal in South Africa which tioday is almost entirely transformed by agriculture and urban development.
This species was last found in 1895 and is now considered extinct.
Purkyn’s Hawkweed is one of about 23 endemic plant species that are restricted to the Krkonoše Mountains, a mountain range located in the north of the Czech Republic and the southwest of Poland; this species occurred only at the Czech side of the border.
The species was a member of the Eginenthal Hawkweed aggregate (Hieracium gombense Lagg. ex Christen. agg.) (see photo).
 Milan Chytrý; Jiří Danihelka; Zdeněk Kaplan; Petr Pyšek: Flora and Vegetation of the Czech Republic. Springer 2017
The Napo Fleabane was described in 1851, the species was only ever found once, growing in wet wasteland at an elevation of 4000 to 4500 m somewhere at the foothills of the Antisana volcano in the Napo Province of Ecuador.
The species was never found since and might well be extinct.
The Lost Sunflower is an enigmatic species only known from a single specimen that was collected in 1851 in Cibola County in western New Mexico, USA.
The species was superficially quite similar to the Common Sunflower (Helianthus annuus L.), from which it differed by its rather narrow linear leaves; it was annual, reached a size of about 90 cm and had a single large flower with yellow florets.
The Lost Sunflower may be an extinct species, however, there is the possibility that it might turn out to be identical with the Pecos Sunflower (Helianthus paradoxus Heiser), itself an endangered species.
The Long-stoloned Mouse-ear Hawkweed is, or was, a unfixed natural occuring hybrid that was restricted to two localities, the Mittelberg near Tegernheim as well as the Scheuchenberg near Donaustauf, both in Bavaria, Germany.
Gay’s Knapweed was described in 1849 based on material that had been collected somewhere in the hills of the Coquimbo Province, Chile.
The species is furtermore known to have also occurred in the Atacama region of the Huasco Province. 
I was not able so far to find any furter information about this enigmatic species.
 Diego Penneckamp; Rodrigo Chaura; Gloria Rojas; Tod Stuessy: Taxonomic notes on Plectocephalus (Centaurea s.l., Centaureinae, Asteraceae) from Chile, including new combinations and synonyms. Phytotaxa 437(4): 227-236. 2020
The Forest Pamakani is a rather tall, upright growing shrub that may reach hegths of up to 2 m. 
The nominate race of the species is thought to have been found on the island of Kaua’i, the northernmost island of the Hawaiian main island chain, this taxon, however, is known exclusively from a single collection and the exact locality has never been recorded.
The only other subspecies (Tetramolopium consanguineum ssp. leptophyllum (Sherff) Lowrey) is endemic to the island of Hawai’i, the species does not occur on any of the islands between Hawai’i and Kaua’i, making the record from Kaua’i very questionable.
The nominate race was never recorded since the type collection and is now considered extinct. 
 Timothy K. Lowrey: A biosystematic revision of Hawaiian Tetramolopium (Compositae: Astereae). Allertonia 4: 325-339. 1986
The Foothill Fleabane is known only from material that was collected during the years 1892 to 1900, somewhere at the foothills of the central Sierra Nevada Mountains in Mariposa County of California, USA.
The species was for some time treated as a synonym of Hartweg’s Fleabane (Erigeron foliosus var. hartwegii (E. Greene) Jepson), from which, however, it differs markedly.
The Foothill Fleabane is thought to be extinct, the reasons for this, however, appear to unknown. 
 Guy L. Nesom: Revision of Erigeron sect. Linearifolii (Asteraceae: Astereae). Phytologia 72 (3): 157-208. 1992
The Sand Pamakani occurs on the islands of Hawai’i and Maui, Hawaiian Islands, with the nominate race occuring on both islands and the subspecies, discussed here having been restricted to Maui only.
The Loose Sand Pamakani is considered extinct, the same was thought of the nominate race, which, however, was rediscovered in 1989. 
 Timothy K. Lowrey: A biosystematic revision of Hawaiian Tetramolopium (Compositae: Astereae). Allertonia 4: 325-339. 1986  Patricia P. Douglas; Robert B. Shaw: Rediscovery of Tetramolopium arenarium Subsp. arenarium var. arenarium (Asteraceae: Astereae) on the Pohakuloa Training Area, Hawaii. Annals of the Missouri Botanical Garden 76(4): 1182-1185. 1989
This remarkable highly specialized plant species was endemic to the Democratic Republic of the Congo, where it was restricted to copper-rich soil.
The species disappeared, together with several other highly specialized species, due to habitat loss by copper mining. 
 Michel-Pierre Faucon; Arthur Meersseman; Mylor Ngoy Shutcha; Grégory Mahy; Michel Ngongo Luhembwe; François Malaisse; Pierre Meerts: Copper endemism in the Congolese ﬂora: a database of copper afﬁnity and conservational value of cuprophytes. Plant Ecology and Evolution 143(1): 5-18. 2010
The Seychelles Vernonia was endemic to the island of Mahé, Seychelles Islands, it is known only from the holotype, which had been collected in 1874 in the so called Forêt Noire, an area that appears to have been largely deforested since then.
The Seychelles Vernonia was a very small shrub, reaching only about 1,2 m in height.
The species was never found since 1874 and is clearly extinct.
 Justin Gerlach: Red List ing reveals the true state of biodiversity: a comprehensive assessment of Seychelles biodiversity. Phesluma 20: 9-22. 2012
The slit-leaved Alepidea is only known from the type material that was collected in 1910 at Gaika’s Kop, a 1963 m high, isolated elevation in the Amathole Mountains in the South African Eastern Cape Province.
Various searches for the species have failed and it must be considered likely extinct.
The Greensword was restricted to the eastern slopes of the Haleakala crater on eastern Maui.
The plant, just like all the other species of the genus, grew as a rosette of numerous long, narrow leaves for several years, until, when finally full-grown, it produced its spectacular inflorescence, just to end up dead.
The species, when flowering, reached a heigth of about 1,5 m.
The Greensword disappeared because of the destruction of the vegetation caused by free-running cattle and sheep – the last individual was found in the year 1945.
There seem to be some plants which may be hybrids of this species and of the Maui Silversword (Argyroxiphium sandwicense ssp. macrocephalum (A. Gray) Meyrat), however, their true identity seems to be debatable.
Saint Helena Gumwood (Commidendrum robustum ssp. gummiferum)
The genus Commidendron consists of only four species, all of which are restricted to the island of Saint Helena.
The 7 to 8 m tall Saint Helena Gumwood (Commidendrum robustum (Roxb.) DC.) is Saint Helena’s national tree and probably was once by far the most abundant tree on the island.
The species can be split into two subspecies, of which the ssp. gummiferus is extinct today. However, there remains a population of hybrids at Peak Dale in the island’s southwestern center, in which this subspecies is at least genetically contained. The time of its extinction seems to be unknown.
… but of course the reasons ….
T. Cavendish in his report about the island (The Prosperous Voyage of M. Thomas Candish esquire into the South Sea, and so around about the circumference of the whole earth, begun in the yere 1586, and finished 1588.) wrotes:
“There are in this yland thousands of goates, which the Spaniards call Cabritos, which are very wilde: you shall see one or two hundred of them together, and sometimes you may beholde them going in a flocke almost a mile long. Some of them, (whether it be the nature of the breed of them, or of the country I wot not), are as big as an asse, with a maine like an horse and a beard hanging downe to the very ground: they will clime up the cliffes which are so steepe that a man would thinke it a thing unpossible for any living thing to goe there. We tooke and killed many of them for all their swiftnes: for there be thousands of them upon the mountaines.”
Those trees, that escaped the goats’ appetite, were later cut down for fire wood.
 Q. C. B. Cronk: The decline of the St Helena gumwood Commidendrum robustum. Biological Conservation Vol. 35(2): 173-186. 1986
The Tasmanian Fireweed was described in 2004; it was very likely long extinct at that time.
The species was endemic to the island of Tasmania, where it was found at an unspecified place in the northern Midlands; it probably grew at lowland plains near swamps; it was actually last recorded in the mid 1800s and is now very likely extinct. 
 Mark Wapstra; Ian Thompson; Alex Buchanan: An illustrated and annotated key to the Tasmanian species of Senecio (Asteraceae). Kanunnah 3: 49-93. 2008
Oeno Island Beggartick (Bidens hendersonensis var. oenoensis)
The Henderson Island Beggartick is found on Henderson Island, Pitcairn Islands and also occurs or occurred on the nearby Oeno Atoll, some three variaties are sometimes distinguished, with the form from the Oeno Atoll being one of them.
The Oeno form was only collected once in 1991, and was probably last seen in 1997.
The Oeno Island Beggartick is now sometimes treated as a synonym of the nominate form, I’ll mention it here just for the sake of completeness.
The Clubmoss Everlasting, also known as Table Mountain Daisy Bush, was restricted to the Table Mountain area in the Central Highlands of Tasmania, Australia.
It was a slender, branching shrub between 0,5 to 1 m tall, the branches were covered with tiny hairs. Its daisy-like flower-heads, consisting of eight to 12 single flowers, appeared in dense clusters at the ends of the branches, they were creamy-yellow.
The species was not found since 1849 and is believed to be extinct.