@zkat you'll have a way more limited pool to get search results from but that's kind of the point. If there's truly such a need for a group of people to be searchable, they can opt-in and find each-other.
I also don't really have a problem with instance-local search when scoped to what the user could've already have seen (Searchtodon had mostly the right idea there). The big problem with Searchtodon is that it holds a centralized index across all it's users.
Instances already have this data, it's in the conceptual security model, it's the addition of an extra (centralized) party that worries me here