User blog comment:MisterWoodhouse/Social Activity, the 2010 Editor, and Unpausing Migrations/@comment-39885705-20200925231410/@comment-26939652-20200926141849

You probably have not enabled the source editor in your settings yet. There's "Visual Editor Source mode", which corresponds to "Source editor" on Legacy (because, why not?), and then there's "Source Editor" on UCP, which is the 2010 source editor, which is still not the classic editor, but also not as horrific as the 2017 editor.

The 2010 editor has some extra vertical real estate, you might be able to get rid of it using CSS or cosmetic filters that exist in ad blockers.