Katello and katello-agent are two very different things!
katello-agent is the client you install on the host to run remote package actions. It is indeed deprecated, but not removed yet. It was supposed to be included in the EL9 tools and I think was left out due to an oversight which is now being rectified (cc @ehelms - does this also apply upstream?)
Katello itself is most definitely not deprecated. It will eventually run on EL9.