It's been a Beta Feature for years without significant change (which normally would mean it should have been discontinued). The product works quite well (and would have been somewhat helpful with a big media story last week, for instance).
The performance impact of enabling this for readers is trivial until the user clicks the "visual" button, which I don't propose to make default yet.
When the long-planned multiple-diff-tools project (to provide single-column diff on desktop //etc.//) happens this could plug into that, but let's just get it out there?
== September 2022 plan ==
* Default "on" for everyone at the wiki (=adds a switching button to diff pages)
* Default to "wikitext" for everyone (it's a sticky pref, so whatever you used last time is what you'll get next time)
* Not Wiktionaries (templates) or Wikipedias (scale/size) during the first round