User blog comment:MisterWoodhouse/Social Activity, the 2010 Editor, and Unpausing Migrations/@comment-7891699-20201112054423/@comment-1869454-20201112075537

To be clear:

"Legacy" Fandom sites run Mediawiki 1.19, which offers the 2006 code editor. That editor was available right through up until Mediawiki 1.32, which removed it from the platform - because the 2010 code editor extension had become available.

Sites switched to "UCP" are based on Mediawiki 1.33... which Fandom rolled out without the 2010 editor, thus leaving no other options but the (rather poor) visual editor. The above announcement marked the point where Fandom finally caved and installed the 2010 extension - but this requires at least Mediawiki 1.31.

Thus there's no way for the 2010 editor to be "made a side-by-side option for all users on all wikis" - nor so for the older 2006 editor, if that's the one you happened to mean.