Is there mention of this in any of the dialogs that one is asked to click "I have read and understand" that were not actually read? I remember Photos telling me that it was scanning the library to do their new layout after whatever update that was. That was to that specific app, and not a system wide notification, but it also said it would be paused until it was plugged in.

I could see a new something in the Settings that indicates post-OS upgrade showing migration status being useful.

If there was I didn't see it. But that is no surprise. Its not a way for them to escape a notification either imo. Especially if its slowing everything down.

That's potentially a good idea. It could even be hidden in the developer section, since those are the people most likely to notice.