User blog comment:MisterWoodhouse/The first migrations to the Unified Community Platform/@comment-7891699-20200813051632/@comment-1869454-20200813110148

Although I'm not aware of fixes or workarounds for most of your issues (and indeed, would like to hear some!), I can tell you that categories can be manually added to pages by simply entering their markup into the source editor (ideally right down at the very bottom of the page code).

Eg:

You may or may not find that you need to tick the "Disable Category module" box on Special:Preferences for pre-existing tags to show up in the source window. That's been bugged pretty much forever, not sure if UCP has finally fixed it.

(I suspect it has, if only because the category module probably can't run at all anymore...)