I wrote a terminal emulator for the Apple ][ + Videx 80 column card in FORTH and reverse polish notation 6502 code, that supported SUPDUP including the line saving protocol that RMS hacked into ITS and EMACS (%TDSAV %TDRES), so Emacs could stash lines on the screen in a memory buffer (on the Apple's bank switched RAM card) and restore them later to paint the screen quickly when scrolling back and forth. (That was great at 300 baud!)

This is just the part of it that interprets SUPDUP codes -- layered on top of the rest of the terminal emulator that emulated H19 (VT52 + character/line insert/delete):

http://www.donhopkins.com/home/archive/forth/supdup.f

Marc Crispin (the author of the SUPDUP RFC #734, who was famous for his catchphrase "MM is not at fault!") also wrote another RFC #748 documenting the Telnet Randomly-Lose Option, which was useful for fuzzball testing.

https://en.wikipedia.org/wiki/Mark_Crispin

https://tools.ietf.org/html/rfc748

   IAC WILL RANDOMLY-LOSE

      The sender  of this command  REQUESTS  permission  to, or confirms
      that it will, randomly lose.

   IAC WON'T RANDOMLY-LOSE

      The sender of this command REFUSES to randomly lose.

   IAC DO RANDOMLY-LOSE

      The sender  of this command  REQUESTS that the receiver, or grants
      the receiver permission to, randomly lose.

   IAC DON'T RANDOMLY-LOSE

      The command sender DEMANDS that the receiver not randomly lose.