Board Thread:Support Requests - Getting Technical/@comment-35086773-20190903192457/@comment-34888194-20190903193750

There the SourceEditButton script which adds a source editor button on the edit drop-down.

If you only want editing in source mode, you can head to your preferences and go to the tab called Editing > Preferred editor and choose Source editor.

Editing only in source editor is not allowed site-wide, as it would violate the Fandom's customization policy.