I'm updating an old article that had used sections/prompts I no longer need. When I cleared those sections and disabled them, they kept reappearing when I disabled another section or continued working on my article (they'd reappear upon autosave). I eventually found I needed to double-disable them: disable them via prompts once so they disappear from the screen, then re-open prompts to disable them a second time before doing anything else. I also tried with a field I'd never used (Subheading) and found the same behavior. I'm using Vivaldi browser (Chromium-based) and working on an old Geographic Location article. I haven't tested in other browsers, other templates, or other articles.
I've seen this, too. I've had to go into the Old Version Edit and clear out the field to get it to stop re-appearing. I'm on manual save and have also found that removing the content, saving, and then removing the prompt and saving again also works to avoid the issue.
I see this in the NEW editor. I can disable the sidebar in Config, but even after reload I can still see the right sidebar.