There are definitely isolated instances that deliberately don't federate with anyone (and thus operate like a typical centralized social network). There are als

There are definitely isolated instances that deliberately don't federate with anyone (and thus operate like a typical centralized social network). There are also widely blocked instances that seem to federate with each other and might effectively form an enclave. It's hard to say in absolute terms. Say the vast majority of instances block instance A, and instance B goes online and begins communicating with the larger fediverse without blocking A (either deliberately or because they aren't aware of A). If users on A and B don't initiate communication, instances A and B won't communicate to my knowledge†, but there's nothing blocking communication. So at that moment A is not totally cut off from the fediverse. †There might be some discovery features I'm not aware of. Also, there are some edge cases. If a third instance C joins the fediverse without blocking A (like B did), a user on C could follow a user on A, then a user on B follows that user on C, and finally that user on C could boost a post from A onto B's timeline, which I believe would cause A and B to start communicating.