curl -sS -X POST 'https://matrix.example.com/_synapse/admin/v1/purge_history/!abc123:example.com' -H "Authorization: Bearer $ACCESS_TOKEN" -d '{"purge_up_to_ts":1735689600000}'

WARNING: deletes room events from the database, reclaiming disk space but permanently removing history before the given millisecond timestamp. Events from remote users are removed while local users' events are kept unless you also send delete_local_events. At least one message per room is always preserved. The returned purge_id lets you poll progress.