Eliminate $wgUseMediaWikiUIEverywhere and the associated code forks entirely, leaving the 'on' behavior.
Note that these are used on mobile and without the mw ui classes the elements appear unstyled on Special:Preferences, Special:RecentChanges and various other form based layouts. So just plain removing them would be a serious css regression that would have to be consider.
The point of this task is to eliminate the global after setting it to true, keeping the true behavior. But we need to decide what we want to do; see T99827: Meeting: MW UI and OOjs UI/OO UI PHP planning.