They have warned about this before, but it’s still pretty wild to think they’re now removing old DMs (and chats, but it’s not like anyone used those).
Interestingly you can still see them if you take out your data via a request, so they got them in the database still.
Unsaid part of that adage was “…in the public”.
People forget about nefarious corporations keeping privately shared stuff indefinitely even if you explicitly tell them not to. Or if they swear it’s getting deleted.
Also, another part of the adage that is incorrect is a corollary to Murphy’s Law. “If you put something online publicly and you expect it to stay online forever, it’s not going to, usually due to corporate bullshit.”