Except you can already query/set the color palette to arbitrary RGB values with control sequences (at least with xterm, rxvt-unicode, and I imagine others).
As far as I've been able to tell, "truecolor" terminal support is about simultaneous display of a range of colors beyond what fits in a palette. Which, eh...seems a bit much for a terminal.
Although there's some real gems in there (I can't believe I never ran across vipe before!), its Debian package conflicts with the GNU parallel package: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=718816 Quite annoying.