To my understanding, there is no difference between Ansible, Puppet, and Salt as far as support in Foreman+Katello on EL goes. I cannot speak for Foreman without Katello or Foreman on Debian.
Regarding documentation: I don’t know why it says “unsupported” right now but I’ll investigate. The last two stable releases are supported; that is Foreman 3.6 and 3.7 as of August 2023.
Thanks for bringing this up. The other reason is that there are standalone guides for “Configuring Hosts with Ansible + Puppet”, but not Salt. Salt is currently part of the “Managing Hosts” guide. I’ll add this to my TODO to extract it to a separate guide.