Would be similar to the Facebook privacy feature where your content is only visible to your friends. The problem with Facebook is they took the granularity of their privacy features to a point where it's a tangled, disorganized web of settings that's difficult for some users to navigate.

Nostr clients should have a simple toggle switch restricting visibility to your follows, and relays should respect this policy by only serving your content up to requests signed by nsecs associated with the npubs on your follow list.