Seems like convincing everyone in nostr to use the same exact specs would be a challenge. What if we come up with a system that doesn’t require everyone to use the same specs? We declare a Decentralized List (kind 9998 per the custom NIP, linked below), called “Bloom Filter Specs”, and list the requisite parameters as “required” tags (rounds, salt, etc). So if you want to use some particular bloom filter, you declare an item on that list (a kind 9999 event) with your choice of specs and then refer to that event wherever necessary.

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Not everyone, just those using the specific kind. The main issue is that there are thousands of potential hash algorithms to use in bloom filters. If we leave it open, all clients must implement ALL hashing functions just so that things don't break when they suddently face a hash that they don't support. Which, in the end, means that nobody can declare full support to the NIP because there are always new algos. Also, there is not really a huge gain from tweaking hashing algorithms for Bloom filters. We just need one that works and everybody will be happy. Just like we only use SHA256 on nostr events.

Have you actually implemented this somewhere? (Production or testing) I’m curious to know what use cases we might expect to see in the wild in the short term if a bloom filter nip were to exist.

Internally yes (I have not saved into events yet). All my event, address and pubkey relay hints are saved as 3 massive bloom filters. So, when the app needs to figure out which relays have given IDs, it checks the filter. Which means that I don't need to save any of the events.