matrix-spec/content/rooms/fragments/v11-redactions.md

Upon receipt of a redaction event, the server must strip off any keys not in the following list:

• event_id
• type
• room_id
• sender
• state_key
• content
• hashes
• signatures
• depth
• prev_events
• auth_events
• origin_server_ts

The content object must also be stripped of all keys, unless it is one of the following event types:

• m.room.member allows keys membership, join_authorised_via_users_server. Additionally, it allows the signed key of the third_party_invite key.
• m.room.create allows all keys.
• m.room.join_rules allows keys join_rule, allow.
• m.room.power_levels allows keys ban, events, events_default, invite, kick, redact, state_default, users, users_default.
• m.room.history_visibility allows key history_visibility.
• m.room.redaction allows key redacts.