Starting March 11, 2020, new wikis use the Unified Community Platform (UCP). Fandom has also started to convert some older wikis to UCP. UCP wikis currently have modified functionality, and are missing some legacy features. Fandom is continuing to work on adding/porting some missing features. For a user-compiled list of bugs, please see this page.
According to the most recent update, the issue with preferences resetting should be fixed. But yes, if you are looking for specifically the classic editor that you see on wikis such as Community Central, then you are out of luck. It simply does not exist on UCP wikis. However, there is still source mode.