It's more than likely that there's a pre v21 exploit that can be used to compromise post v21 clients, and that's probably why they are kept separate. Blockland still has some sort of memory overflow exploit which allows people to insert stuff into the engine while its overflowing, so it's probable that the older versions of blockland had more engine exploits and v21 secured them, but the clients may still be suceptible to some sort of backdoor