Speaking of telnet, that protocol was actually much richer and had many more features than it would seem at a casual look.

I learned that when implementing a tiny part of the telnet protocol in a retro computer emulator's serial port emulation routines, mostly just to enable character mode and proper (non-)echoing. (I also implemented the RS232 control lines, so that telnet'ing into the port actually behaved like a physical terminal would with respect to carrier detect and all that, but I think there wasn't any overlap with the telnet protocol itself. Maybe flow control.)