matrix-spec-proposals/event-schemas/schema/v1/m.room.history_visibility

{
    "type": "object",
    "title": "Controls visibility of history.",
    "description": "This event controls whether a member of a room can see the events that happened in a room from before they joined.",
    "allOf": [{
        "$ref": "core-event-schema/state_event.json"
    }],
    "properties": {
        "content": {
            "type": "object",
            "properties": {
                "history_visibility": {
                    "type": "string",
                    "description": "Who can see the room history.",
                    "enum": ["invited","joined","shared"]
                }
            },
            "required": ["history_visibility"]
        },
        "state_key": {
            "type": "string",
            "description": "A zero-length string.",
            "pattern": "^$"
        },
        "type": {
            "type": "string",
            "enum": ["m.room.history_visibility"]
        }
    }
}