There is no notion of moderation in ActivityPub itself AFAIK, but I suppose you're asking what happens to e.g. comments that have been sent to federated servers/software and then deleted on the original platform.

In general, when that happens the comment is removed on the host platform, then a deletion request is sent to the federated platforms. There is no guarantee that other platforms will delete the content (although if they don't, they are generally considered malicious and should be reported), much like deleted forum posts can be viewed through WebArchive.