Damus doesn't have search integrated, which makes a third of Nostr users think it's completely broken. Amethyst has our relay for search by default, last time I checked it worked pretty well. Primal has their own search. There is search by nostr.wine relay and also relay.noswhere.com. I recently saw a rust-based relay with full-text search.

Search infrastructure is available to any app, not all of them care enough - lack of real demand, most likely.