For the last six months or so, my Mastodon instance has not been able to show previews of YouTube links. I upgraded to the most recent release (v4.3.2) in hopes that it will have been fixed. It was, for "normal" YouTube links, but still not for shortened ones (youtu.be links).
I was grateful to find this isolated fix to the problem, which just involves changing Mastodon's link scraper user agent: