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Of course, there is probably a large number of currently unlisted videos from accounts that are no longer active, which would effectively be lost after this change.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.
It'd be interesting if Google had made this only apply to accounts that have had activity in, say, the last six months. If an account logs in and was skipped due to inactivity, it would then be appropriate to prompt them for their decision.
That, of course, requires significantly more engineering so I can see why it didn't happen.
Of course, there is probably a large number of currently unlisted videos from accounts that are no longer active, which would effectively be lost after this change.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.