xterm is on that list (and at least for me, the distro provided version has sixels enabled, I've not dug into what build options can be switched off, but I wouldn't be surprised that lots of things that people assume, such as colour depth, can be switched off). Given the vt52, vt100, vt102 and vt200 predate the introduction of sixel (based on https://en.wikipedia.org/wiki/Sixel), I'm not surprised those 40 year old terminals don't support sixels ;)
