[abcdef123xyz123]

My point is that for new features, like key up would be, it is not necessary to be bound by terminfo. If some or all of xterm, VTE, konsole, rxvt-unicode and iterm2 supported it, that is a large proportion of the modern *nix terminals that people still use. And they are all VT100 compatible terminals. If you look at a lot of recent features, such as SGR mouse mode, RGB colour, underline styles, focus events - they are all based on VT100 style escape sequences, and they all started with a few terminals and existed for a good while (with applications using them) before terminfo added any sort of support.