This post on the Flarum site recommends clearing the forum cache, I don’t know if that’s been tried: https://discuss.flarum.org/d/33807-youtube-embed-video-not-working
If that doesn’t work, I found this (old) post with a similar problem: https://discuss.flarum.org/d/31548-embedding-youtube-videos-since-upgrade-and-se9-media-embed-removal/4
Which led me to this site that uses some version of Flarum (link not for clicking): https://www.onemickjones.com/d/105/283
The page is just embedded Youtube music videos (that work).
The last embedded video on that page, posted 3 days ago, does not work here.
From this I’m assuming it’s not a general Youtube or Flarum issue, but an issue with our implementation of Flarum.
(No hurry on this stuff, SLC, just trying to collect information from the internets. If some music videos don’t work, I’ll live with it.)