(Via EqD.) YouTube has announced that, as of 23rd July, all Unlisted videos that were uploaded before 1st January 2017 will be set to Private. In other words, they will become completely inaccessible to anyone but the uploader, regardless of whether or not you have the direct link. As far as I can see, the only exceptions will be where the channel owner personally opts out between now and then. If they don't do that, then kaboom: no more public access.
It's hardly news that, when MLP creators moved away from the fandom over the years, some of them preferred not to leave their Pony videos public for a variety of reasons. However, quite a few did set said videos to Unlisted instead of Private, thereby allowing access if you had the direct link. This often happened if the artist was okay with access from within the fandom but not with people stumbling on their old MLP videos accidentally.
Needless to say, this news has caused a bit of a stir. There are a heck of a lot of Unlisted videos in the fandom, some of them pretty big. "Flutterwonder" was Unlisted for quite a while, for example, although it's been Public again for a long time now. And a number of channels are now unattended and inactive, so the chances of their creators logging back in to opt out are very low. In under a month, those videos will be no more as far as the fandom is concerned.
Sure, archive channels exist, but those are morally ambiguous in that you don't have the same confidence that the creator is okay with fandom access that you do when they've kept a video Unlisted on their own channel. A link from an old EqD post that points to the creator's own now-Unlisted upload will also not tell you where to find the same video in an archive. Plus archives quite often overlook more obscure and/or specialised works.
Some things will be saved. Perhaps quite a lot, given this fandom's continuing size and activity levels. But quite a bit of fandom history will disappear forever on 23rd July. Regardless of YouTube's reasons for making this change (which seem to be to do with the security of Unlisted links) it's certainly going to be a rather sad day.