It was a bit of a shock when I upgraded my Debian systems to Bullseye (11) from Buster (10) a year or so ago. I discovered that the wiki I've been using for years, Moin Moin, was removed because it didn't support Python 3. I've a lot in the wiki and was in no position to migrate, so I made a wiki only VM and keep it at "oldstable". Since none of it is internet-facing, I can keep it this was forever, even once all support ends. I'd rather not have to have another VM though.
MoinMoin2 exists and works on Python 3, but sadly there's still no stable release and it requires slightly older Python packages right now. We've also had strange problems with index (Whoosh library) performance on a large wiki that we had to hack around, but it might be a good migration path for you.
And if you don't want to use moin2, it can convert the custom markup to Markdown, which you can then use elsewhere.
It looks like the MoinMoin people decided to rewrite a lot of stuff for Moin 2 (which does support Python 3), but that hasn't been finished yet and has been in the works since about 2010.
I had a VM with MediaWiki way back when and I was able to convert it to markdown and a private GitHub repository pretty easily with pandoc. I'm not familiar with Moin Moin but maybe there's a conversion tool you can use to do the same thing?
