very few, and none of the major ones (primal is working on it though and  nostr:npub16c0nh3dnadzqpm76uctf5hqhe2lny344zsmpm6feee9p5rdxaa9q586nvr mentioned support for outbox should be rolling out next week)

essentially it boils down to:

a) use a static list of relays (what most clients do)
* then if someone publishes something in a relay not in that list you won't see those events

b) use the outbox model (or something like it) where your client follows instructions the people you interact with provide to find the content you are interested in in their respective relays

nostr:naddr1qqxnzde3xy6rvwpnx56rvdpkqythwumn8ghj7un9d3shjtnswf5k6ctv9ehx2ap0qgspwwwexlwgcrrnwz4zwkze8rq3ncjug8mvgsd96dxx6wzs8ccndmcrqsqqqa28smwra2