Correct. This is something I and several others have been doing for some time now. We have a private search engine that covers most of the Mastodon fediverse (including widely defederated instances), and there's nothing anyone can do about it.
Eventually, we'll give this data to the Internet Archive or put it in a torrent or something. It includes a decent amount of now-deleted content too.
Really, it's no different to what others, e.g. Pushshift, are doing with other sites. Except there's no opt-out. Anything you've already said is almost certainly in our data set, and it's there permanently.
> That’s extremely hard to prevent but isn’t really the problem: The problem would be a public search engine that Gamergaters and Kiwifarmers use to hunt down vulnerable targets.
Also correct. A couple of KFers we know already have access to search the data we've collected. It points them towards interesting posts or accounts to archive. People say some pretty wild things when they think they're not being recorded for posterity!