This intervention changes the nature of a subscription based site by unifying two actions "user A subscribes to user B's posts" and "user B subscribes to user A's posts". This form of limitation is most often expressed through the semantics of "friend requests". Another way of framing this requirement is the omission of the feature of one-way subscription.
Requiring subscriptions to be reciprocal has a number of advantages to user safety, making it harder for folks to Stalk or Doxx one another. Its larger effect, though, is to preference social relationships (a relationship between two people) over parasocial ones (a relationship between a creator and an audience). Similar to Limit Number of Subscriptions, this approach would make it harder for influencer-dynamics to take hold, and would direct user attention and energy toward their real-world relationships.