It’s hard to commit to any version.
@akumari has done a good job on making sure the Smart Proxy is tested with Ruby 3 in our CI. While it’s not acceptance tests or full system integration, it is a solid base layer.
As for Rails, it’s less positive. While my PR (Rails 7 & Ruby 3.1 by ekohl · Pull Request #9328 · theforeman/foreman · GitHub) might give the impression it’s close, that’s only the start. Once we have Zeitwerk working in Foreman itself, we must also make sure all plugins work with it. That hasn’t even started so who knows which dragons lurk there. I’m trying to work with @ofedoren and @akumari to make progress, but given where we are I wouldn’t expect it in Foreman 3.9.
For EL9 we also need to tackle our NodeJS update, which implies a webpack update. @MariaAga has submitted https://github.com/theforeman/foreman/pull/9834 which is a leap forward. Resurrection of the client-side infrastructure upgrade effort is also tracking that. AFAIK that has been tested in developer environments, but is now in review and packaging also needs to be done. Given its current status this has the potential to make it into Foreman 3.9.