I'm not sure if it'd even be possible either without the relay forcing the REQ to include the requestors own pubkey in order to see if it has been muted by the other pubkey and therefore restricted from fetching the other person's notes. Would add a lot of additional overhead imo. Which is likely why most have defaulted to letting the client handle that side of things. Though, perhaps there could be a NIP that's used across clients/relays where a new kind (like a mute list, similar to follow list) is stored that clients could use and determine how to handle blocking those who have been muted. That way, one user on Damus can mute/block someone and that someone no longer sees those notes on Amethyst.