We need a way to hide given messages from specific room members (SPEC-84)

If you're trying to loadbalance messages between multiple users in the same room, you may want to make the message only visible to subsets of the room at any given time.

This is similar to ACLing off historical message visibility if you so desire.

(Imported from https://matrix.org/jira/browse/SPEC-84)

(Reported by @ara4n)

Start by reviewing the imported SPEC-84 request and the surrounding Matrix specification to identify where room message visibility is defined. Clarify how subsets of room members and historical message access should work, then document the protocol behavior and its completion criteria.

Written by the indexing model from the issue text.