Most distros use https://github.com/SELinuxProject/refpolicy while RHEL uses https://github.com/fedora-selinux/selinux-policy

It's also my experience that Fedora has better support for it, but Gentoo used to be good enough with hardened gentoo (they use https://gitweb.gentoo.org/proj/hardened-refpolicy.git/). Redhat and Gentoo are the only ones that officially support it afaik. I think hardened gentoo might have lost popularity since the fall of grsec, but I'm not sure how popular it is currently.