vlemmy was on a free .ml domain but unfortunately the .ml top-level domain got taken back by the Mali Government. Same thing happened to lemmy.fmhy.ml which was pretty big too.
Are you thinking of the mastodon based Kolektiva.social? The server hosting it was apparently seized by the FBI for an unrelated raid on the owner of the instance. There wasn't anything related to CSAM in that situation though
The server wasn't raided, the person being raided had an unencrypted backup of the instance database on their personal computer, which got seized in a raid unrelated to the Mastodon server.