User blog comment:MisterWoodhouse/The first migrations to the Unified Community Platform/@comment-2256018-20201006175328/@comment-47011541-20201006175950

The UCP is a new platform and therefore some features weren’t able to be included with it:

Chat and Forums were features that wouldn’t have been worth having with the new platform, they were respectively succeeded by Discord (and other chat systems) and Discussions (which was in usage as an replacement for Forums around 2016/17)

WikiActivity and Wall Greetings were reworked into scripts (WikiActivity and WallGreeting respectively).

The rest of the features are currently in an unknown position or have yet to be transitioned.