BuddyPress.org

@grosbouff There is the BP Followers Plugin by @apeatling for your reference

One difference between Friends and Followers is that Friends is in core while Followers feature needs the plugin to be activated.
https://wordpress.org/extend/plugins/buddypress-followers/
https://wordpress.org/extend/plugins/buddypress-private-messages-for-followers-only/

The other difference is Followers is ala Twitter following. The “follower” doesn’t need to be approved by the one he follows. While for “Friends” to connect completely friendship request must be approved by the other party.