I believe there is a whitelist mode, that isn’t currently enabled but in that mode new instances are defederated by default. That would be a pain to administer for all the small instances joining, so they may set some size or reputation rules to apply for federation (??). I imagine this is inevitable if automated instance spamming becomes a problem, unless some type of RBL comes in vogue for managing federation first. It may become impossible to manage manually.

E: oh the whitelist thing may only be relevant to kbin federation, as that was the topic I saw it in. Sometimes I forget where I am :-/