User blog comment:MisterWoodhouse/Social Activity, the 2010 Editor, and Unpausing Migrations/@comment-35455167-20201103004626/@comment-1869454-20201103235726

Go to your editing preferences panel and switch from the (frankly rubbish under any browser) Visual Source extension to the dedicated Source editor.

Fandom being Fandom, doing this will force your default editor on non-UCP wikis to Visual, but at least you'll be able to edit UCP wikis again. As much as is possible without visible images etc, at least...