Earlier quoted context omitted.
Did they send an email to affected accounts? There seems to be a logical reason for this (newer unlisted videos have a more secure url generator), so I’d say this is neutral at worst.
Some of my videos would be affected and I got an email from them. Seems reasonable to me. Of course, there is probably a large number of currently unlisted videos from accounts that are no longer active, which would effectively be lost after this change. Unfortunate.
That, of course, requires significantly more engineering so I can see why it didn't happen.