just kbin.social. all kbin instances federate with everyone and block no one by default but instance owners can intentionally defederate/block if they so choose. afaik kbin.social's policy is to not defederate/block (at least the admin here hasn't mentioned anything yet). I haven't heard of other kbin instances defederating at all though, but most are so small to not even be noteworthy.
yeah federating with all by default is just a fediverse thing in general, though some apps my include existing blocklists (idk if any actually do or not).