Oddly enough these things have been going on since 2001, and have been curated gardens instead of the mess that is usenet (which is still operational!), or private mailing lists.
The 2 that Im on are the Hercules 390 group which moved to group.io. While other groups like H390-music (MUSIC/SP the Canadian mainframe OS with internet hooks) that sadly died along with it’s author, is probably going to be purged from the internet.
I don’t know what the answer is, other than to always have downloadable mailing list archives, and never trust a single place. So much stuff is deleted to save trivial amounts of space, and neither corporations nor government institutions can be trusted to maintain anything.
Thankfully there is archive.org, but who backs them up?