Is cpanel only supported on basically 2 different OS's now?

Ubuntu does actually work with Cpanel but I don't recommend it as it is not officially supported at this time and requires a couple of manual modifications to prepare the system for Cpanel installer to work correctly else your install and future Cpanel updates blow up pretty bad.

Redhat / Fedora - Both of these work sufficient enough though tend to be a little bit more buggy than the next choice.

CentOS - Except for the very rare, Cpanel works amazingly well with this operation system and would be my first choice when building a Cpanel base server machine (especially x64 bit if your CPU supports it)