In TNG: Gambit, Tallera/T'Paal was a Vulcan isolationist posing as an undercover Vulcan intelligence agent posing as a Romulan mercenary. She beamed down to the T'Karath Sanctuary on Vulcan with her ridges still on, which could mean there might be a small minority of other Vulcans who have the ridges themselves, like their Mintakan proto-Vulcan ancestors.Honestly, the Romulan change is the only one that really bothers me. They're supposed to be the same species as the Vulcans, so it always seemed weird to me that Romulans got the ridges, but not the Vulcans. Things get especially weird once we throw in Spock going undercover and not being instantly identified as a Vulcan and sent packing.
The episode deals with ancient psionic weapons like the Stone of Gol that target aggressive thoughts, so I wonder if the Romulan faction in the war didn't genetically modify themselves with the ridges as sort of a psionic defense against weapons of that sort. In the process, they also eliminated their own ability to communicate telepathically.
The ridged Romulans were always the grunts and the common folk forced to fight in the war, while the ridgeless Vulcanoid Romulans were the upper class and ruling class. The ridged Romulans were always there, just in TOS they were often in the background under the helmets. Eventually sometime after TOS, the ridged Romulans rose up against their betters and conquered them (possibly committing genocide against them), causing a shift in Romulan society, which is why we almost always see ridged Romulans now.