The Mount Holland Thomasia was described in 1974, based on the type material that apparently was collected in 1929; it was restricted to a region somewhere near Mt. Holland in southern Western Australia.
The species is now most likely extinct. 
 Kelly A. Sheperd: A key to the species of Thomasia (Malvaceae: Byttnerioideae). Nuytsia 30: 195-202. 2019
The oldest known specimen of this species was collected on the island of Tahiti, Society Islands in 1769 during Cook’s first voyage around the world, the species was subsequently collected only five more times with the last specimen having been taken around 1850.
The species was apparently more widespread in the Society Islands, as at least one specimen was found on the island of Bora Bora, this was described as a distinct species in 1981, however, was later downgraded to synonymous status. 
The Tahitian Pavonia disappeared at the middle of the 19th century.
 F. R. Fosberg; M.-H. Sachet: Pavonia (Malvaceae) in the Society Islands. Bulletin du Muséum National d’Histoire Naturelle 4 sér. 3, section B, Adansonia 1: 15-18. 1981
This species was described in 1854, it is apparently only known from the island of Maui, Hawaiian Islands, where it was found growing in sympatry with the very widespread Sleepy Morning (Waltheria indica L.) (see depiction below).
The Maui Sleepy Morning was later synonymized with that species but was subsequently resurrected in 2011; it may, however, indeed be nothing but an aberrant form of the common Sleepy Morning. 
If it, however, indeed was a distinct species, it is now definetly extinct.
 Janice Saunders: Resurrection of the Maui endemic Waltheria pyrolifolia (Sterculiaceae, Hermannieae). Darwinia 49(1): 76-85. 2011
The Dwarf Ebony, also known as Saint Helena Ebony, was one of altogether three species within its genus, all endemic to the island of Saint Helena in the middle of the vast Atlantic Ocean.
The species was a smaller tree with beautiful white flowers, it was once quite common and covered the slopes of its home island; it was among the first plants that fell victim to the feral goats, imported to the island at the end of the 18th century, and who ran amok amongst the island’s endemic vegetation.
This enigmatic species was growing exclusively around the city of Vranja in southeast Serbia.
The species disappeared (when?) after the destruction of parts of its habitat due to urban development by the city of Vranja, and the conversion of the remaining parts for agriculture purposes.
I have to admit, however, that the exact status of this plant (accepted species or synonym) seems not to have been clarified yet.
There is another form of marshmallow, the Kragujevac Marshmallow (Althaea kragujevacensis Pančić ex Diklić & Stevan.), which was thought to be restricted to the vicinity of the city of Kragujevac in Šumadija, Central Serbia.
This form was long thought to represent a distinct and now extinct, endemic Serbian species, but is now, however, treated as a synonym of the Common Marshmallow (Althaea officinalis L.).