2017-02-20 22:30 GMT+02:00 Juliusz Chroboczek <jch@irif.fr>:

I've only had a quick look at the code, but it's my understanding that
the feature is only enabled if terminfo has non-standard properties
setb24 and setf24. Since it's easier to add a line to .emacs than
compiling a new terminfo, I'd like a way to enable the support without
hacking terminfo.

Using terminfo was the easiest way to implement this feature. I was told earlier that the terminal-parameters alist could be useful for implementing the configuration from elisp, but I never bothered to investigate that option further.

I'm definitely getting only 256 colours in a recent Debian testing XTerm.

XTerm approximates 24-bit colors using the palette of 256 color mode.