Wikis created after March 11, 2020 are using the Unified Community Platform (UCP). UCP has less functionality than pre-UCP wikis and is missing many legacy features. Fandom is continuing to work on adding/porting missing features. For a user-compiled list of bugs, please see this page.
Interestingly enough, the classic editor is still available for MediaWiki 1.32. Fandom is upgrading to MediaWiki 1.33. So if Fandom had decided to go just one MediaWiki version older, they could have kept it.