about compatibility

Curses/ncurses are a UNIX/Linux specific: lots of legacy code from other OS's don't use them at all.

In practice, the VT100/102/220/240 line were mostly faithful to the ANSI standard and very widespread in practice as
DEC terminals were very common (and not just on DEC systems); if you nail them, then you've covered just about anything software out there, as they had to be supported back in the day. And emulating them exactly was more
important than the ANSI standard itself.

The xterm secret sauce was the parser itself, which actually did parsing properly according to the standard. Thereafter it is mostly a matter of simple code and implementation. But it was hard to get the parsing right: I had to deal with that before the mentioned code donation from people in DEC (that I also worked for at the time), who were experts at
it. Life was much easier once someone else solved that problem for me (or lifted the code; as I said, I'm not convinced they should have given the code to me). It is also very possible/likely they may have reimplemented the parser rather than given me DEC copyright code: but they were true experts at terminals who did so, and may not have been properly authorized by management. We got much more careful about contributions as time went on, and DEC is long past history....

I know the actual code base used in DEC terminals was different though; that I am sure (it was on some tiny microprocessor in the terminals, and unlikely in C at all).

In the early X era, having a solid terminal emulator that really "just worked" for almost any application was one
of X's important features. And I'm oh so happy to not be the xterm maintainer (which I was for a few years early
in X...) My immense thanks to those who are....