"That problem was fixed within a day of the time it was reported. "
How would anyone know that? I had the problem in .100, which promised a fix, maybe so but not to the problem I encountered. That seemed worth reporting to JR. There is no mention of long filenames in the Release Notes of .101 or .102, so any further work on this is unknown to users.
The best, or in fact often the only way to know what has been done is via the Release Notes, as you noted. They are summarised and pretty up to date if I am around, here:
https://wiki.jriver.com/index.php/Release_Notes_MC23As an aside I think the original long file name problem was different and was fixed in MC23.0.99, which highlighted another problem which was fixed in MC23.0.100, which is why there was no further reporting of fixes.
I don't think that fixed the problem you saw, which probably still exists, because you were doing something that most people wouldn't do: Changing the directory name via MC's Drives & Devices. That is pretty unusual. I would never have thought to rename a directory that way. I always use the RM&CF function, even though it is slower. It may fall under the category Hendrik mentioned, in that;
But the fact alone that this support is hidden behind an experimental option means that it won't work with all use-cases in MC, and is known to be incomplete.
But I suspect it is a different issue altogether. It was just catching that error message because Windows was telling MC it wouldn't do the move for some reason.
PS:
I suspect that if you have MC set to update for external changes, backed up the Library, shut down all of MC, changed the directory name using Windows Explorer which would be very quick on a local NTFS drive, then started MC and ran Auto Import, the change would have been picked up in MC very quickly as well. I'm not sure if it would be as quick if your files were on a NAS, but it might be. Although every file would still need a tag change, the files wouldn't be copied from one directory to another one at a time, which would have been why it was slow using the RM&CF function. The thumbs.db files wouldn't have been an issue using this method, as Windows isn't moving them individually, just updating the NTFS tables underneath the directory they are located in. Besides, even if the message still appeared, you can tell Windows to apply your decision to move them all by ticking the box.
It is even possible that changing the directory name using Windows Explorer wouldn't have worked, as essentially that is what you are doing by using MC's Drives & Devices. If it didn't work you would be highlighting a problem in Windows, not in MC.
I have used the above method in the past, and it worked fine. But even if it didn't work for you, all you would need to do to recover would be to shut down MC, rename the directory back to the original name, then start MC and restore the Library backup you made. I have also reversed the process this way, when I made a mistake in the original renaming. The only risk would be if some files had their tags updated and some didn't, in which case an Auto Import run should change the tags back again. You would then be back to where you started and could use the RM&CF method.