The kind 9998 list header declaration could specify the hashing algo. Or we could leave the hashing algo unspecified and recognize that it is not necessary for all clients to support all hashing algos, just like it’s not necessary to support all NIPs. Probably the community will gravitate to one algo organically, unless some devs have strong preferences that are not always aligned. If getting everyone to agree to all the details is trivial, is there any reason not to go ahead and write up a bloom filter NIP?