Like all those people who paid for Google music for years and then when it was killed and replaced with YouTube music lost out because some stuff wasn’t on there, or wasn’t available in their country, etc right?

All my purchased music was properly transferred to YTM, and the analogy isn't quite 1:1 since I can download the mp3s I bought too and store them on my computer. There may have been some cases where specific songs licenses weren't available (I'd like to see the percentage), though the help page says that once they get the license, the song will re-appear in your library too. Over the past year I have found their licensing has gotten better and I run much less into missing albums anymore.

Did they actually remove purchased tracks? Or just couldn't stream the same songs? You buy the games on Stadia so the comparison only works if it is the former.

I believe you can still download the mp3, just can't stream it since they don't have the streaming licences, though they have been re-acquiring a lot of licenses over the past year for YTM.