User blog comment:MisterWoodhouse/The Unified Community Platform Editor and our first release date/@comment-24155515-20200309205452/@comment-24006128-20200309210556

We're working on getting that preference sorted. The preference changes how it operates, as the legacy platform has multiple different editors and, with this release, the UCP will have 1 editor with 2 modes.

The 2017 Editor's Source Mode is a source editor.