Thanks, everyone.
1. This behavior occurs both on windows (10.68.3-win-winstore-public) and on the web. The screenshot above is from the web.
2. On Windows, the deleted folder disappears for a moment and then reappears. On the web version, it won't delete and displays the above image.
3. None of these are the default folder.
@agsteele - Given that this occurs both locally and on the web, do you think rebuilding the local data would help? It's a good idea.
@PinkElephant - I'm trying to slowly delete all the cruft so I can move the good stuff to Obsidian. I'm grateful that under the new free plan, I can still edit notes.