Amhoanna, do you mainly mean seal scripts yet also including 正體/楷書 e.g. 纔 -> 才, 臺 -> 台?
What's a seal script? 台 and 才 may be a subset of what I'm talking about... We actually still know 纔 and 臺... We still see these hanji in print from time to time... But most of the iceberg lies submerged, creating this illusion of 正体字 being uniquely 正. Even just on the 異体字字典
http://dict.variants.moe.edu.tw/main.htm -- which doesn't seem to go all the way back to 甲骨 origins -- most characters have so many variants. Over half of the unfamiliar forms are more complicated than our 正体字. I'm guessing they're generally also older. This is just a guess. To the extent (i.e. percentage) that my guess is right, though, our 正体字 are actually 簡体字 themselves.
Is 台 in 臺灣 -> 台灣 considered 簡體 in Taiwan itself?
I think most TWese would consider it to be an alternative hanji, kind of like 線 and 綫 or 館 and 舘. On the other hand, 湾 would be considered 簡体, yet the 湾 form is everywhere, in writing, on khanpáng (shop signs), in ads, etc. ... but in full text, printed or typed, there's almost an instinctive, nationalistic insistence on 灣 as a FUBU marker (for us, by us).