`m.room.history_visibility`

m.room.history_visibility controls who can view the room history.

Example

{
  "history_visibility": "invited"
}

world_readable - anyone can read the history without joining, and it likely will be indexed by search engines.
shared - history is shared to all users in the room, regardless of when they joined.
- Matrix public archive will publish history of rooms with this option although not allow search engines to index.
invited - users can see history from the point of time they were invited to the room. RECOMMENDED FOR SEMI-PUBLIC ROOMS
joined users can see history from the point of time they joined

WARNING! Changing the option will not affect old messages.