Board Thread:Support Requests - Designing Your Wiki/@comment-36535642-20200419144903/@comment-44807868-20200419152549

Yes, the wikis created after November 2016 don't have forum enabled, and loads of communities with forums have been being migrated to Discussions. It's also planned to completely retire in future. Which's all due to robust reasons, of course.

So I'm afraid you're likely out of luck with having it on your wikis.