Someone has apparently written open-source software to circumvent fediblocks by "woke" instance admins (I kid you not -- they're unironically using that term).
The only domain we currently know of providing this service is activitypub-proxy.cf, but I'd expect more to pop up.
We may need a tool to periodically scan federated domains for heavy subdomain usage that could be this software in use.
Anyway, thought everyone should know.
-- your woke administrator (well... partly woke... I haven't had my morning caffeine yet)
We have lots of possible heuristics. For example, a simple heuristic is "federate with instances if they contain a user that's followed by someone followed by one of our users" (i.e. at distance of <=2 in directed graph of follows).
Also, making it costly (applying moderation actions per ~everything-under-same-domain-registration) might be sufficient.