Changing the domain of an established fedi instance is very difficult, almost to the point of impossibility.

They are mastodon-specific, but most fedi software has a similar feature. Or at least, all of the mainstream microblogging software does, as well as some of the image / video sharing platforms. I’m unsure about Lemmy and Kbin. Here are the equivalent settings for FireFish:

Defederating actually does stop Meta from accessing data (at least through ActivityPub) if you enable AUTHORIZED_FETCH / similar. That setting requires remote instances to authenticate themselves, which prevents blocked instances from querying anything. IIRC, Lemmy either already supports or plans to support that same feature.

Meta could, of course, just use web scraping, but that can be prevented with DISALLOW_UNAUTHENTICATED_API_ACCESS. Although admittedly, I don’t think Lemmy has this feature yet.

You’re thinking of LIMITED_FEDERATION_MODE, which is different from AUTHORIZED_FETCH.

Defederation actually does work both ways if the instance enables AUTHORIZED_FETCH. That setting requires 3rd party systems to prove their identity before they can retrieve any data, which allows an instance to block defederated domains. I don’t know if Lemmy or Kbin supports that, but practically all of the microblogging fedi software does (that being Mastodon / GlitchSoc, Pleroma / Akkoma, Misskey / FoundKey / FireFish, and GoToSocial).

I agree that this is nothing to panic over, but I want to clarify that Lemmy is not safe from this. Lemmy and Mastodon both use the same protocol (ActivityPub) and that’s also the protocol that Threads will use to federate. Just as Mastodon users can like, boost, and reply to Lemmy threads / comments, Threads users will be able to do the same. That’s why it’s important to defederate Threads on all ActivityPub-enabled instances.

For single-user Mastodon instances, it really helps to become mutuals (you both follow each other) with some people on a few big instances. That will establish federation between your instance and theirs. Other things that can help are:

• Interact (like, boost, reply) with interesting posts from another instance
• Fill out your profile! That side of fedi tends to distrust blank profiles.
• Write and sticky an introduction post, for the same reason as above.

TIL that Danbooru is decentralized