> Why? In 2017, we rolled out a security update to the system that generates new Unlisted video links. This update included security enhancements that make the links for your Unlisted videos even harder for someone to discover if you haven’t shared the link with them. We’re now making changes to older Unlisted videos that were uploaded before this update took place.
Yes.
Edit: they give the reason explicitly https://support.google.com/youtube/thread/114633828/changes-...
> Why? In 2017, we rolled out a security update to the system that generates new Unlisted video links. This update included security enhancements that make the links for your Unlisted videos even harder for someone to discover if you haven’t shared the link with them. We’re now making changes to older Unlisted videos that were uploaded before this update took place.