I don't know when the term became widespread for gui-style terminal programs, but the wikipedia entry has existed for more than 20 years so I think it is an older term than you imply.
https://en.wikipedia.org/w/index.php?title=Text-based_user_i...
https://en.wikipedia.org/w/index.php?title=Text-based_user_i...
The styling "Terminal" is new to me; I'm not sure I've even ever seen it before this thread. That's the "new-fangled bit for gen-Z", as others in this sub-thread have bemoaned, AFAICS.
And I must admit I rather agree with those bemoaners: It's stupid. Sure, you do run your TUI apps in the terminal, but the terminal itself is really a CLI, not a TUI. This makes the term rather misleading.
So "Text[ual|-based] User Interface" is a far better reading of "TUI" than "Terminal User Interface", IM(ns)HO.