Very little actually. Depending on how the shutdown works in practice, the impact is basically nothing. Other instances just stop receiving updates from the ins

Very little actually. Depending on how the shutdown works in practice, the impact is basically nothing. Other instances just stop receiving updates from the instance, which just results in the users in the closed instance being cached artifacts (zombie accounts) that need to be cleaned out manually. There's also a "self destruct" feature in Mastodon which is the nice way to shut down an instance; it issues account deletion messages for every account to every instance it federates with. The idea being that this results in the federating instances processing the account deletions accurately. As for requests to the original server; basically all instance software (Mastodon included) implement a backoff mechanism, meaning that if after 3 months your server is still returning 404s when requesting new information, the software will quietly stop requesting new info unless explicitly asked to do so by a user.