I have read that people still use 9term. Its terminfo entry doesn't show any sign of ANSI escape code support.

If I remember correctly, ANSI-like terminal emulators in RGB mode (aka direct mode) only support 16-color and red/green/blue escape codes. Not the 256-color palette used in this article.

ANSI-compatible terminal emulators widely disagree on whether RGB values should be separated by semicolons or colons.

I have read articles (possibly on hackaday) this decade by people working on retro hardware projects, expressing frustration with command line programs that spew ANSI garbage to their non-ANSI terminals. At least one author resorted to the screen program, because it can translate some ANSI-isms to the correct codes for the active terminal.

I had genuine TeleVideo 912 glass terminals hooked up until not terribly long ago.

In any case, I don't view this as a matter of whether unusual terminals are "notable". We have an abstraction that plays nicely with them (and allows new terminal protocols to be developed). It's a POSIX standard. It's easy to use. I suggest using it.

(I'll put examples in a separate comment.)