I'm not sure, but what sets have become legal, because I was wondering how far back the unretired sets go.

Every set all the way back to DA should be open to use again, however different scenarios will rule out certain pieces and Arty will probably be getting dumped sometime soon.

At this point, all sets are tournament legal. Certain events will allow the venue or battlemaster to set "Battleforce Restrictions." That way the venue can disallow units based on their local atmosphere.