yeah, mute lists, at least, some clients are putting encrypted blobs in the content field that contains lists of npubs inside it. #jumble lets you do this, there is two mute options, mute publicly and mute privately.

i don't think anyone has made private follow lists or any of the others, but all of them clients mostly disregard the content field unless they implement this feature, so it could be added to all of them.

it's not really the same because it's not discoverable though.