This rules update compiles changes from the linked post as well as those only noted on the Discord server to bring both sites to parity.
Zenith Flare (Ikoria: Lair of Behemoths), Run Away Together (Throne of Eldraine), and Ardent Electromancer (Zendikar Rising) are banned in Primordial.
Wishing, Learning, and Companion Rarity Rule
No rare or mythic rare can be brought into a game from outside the game. This means that players cannot “Wish” or use the Learn ability to choose rare or mythic rare cards from their sideboard, and Companions cannot be brought in by paying for their ability. Players are still able to sideboard in rares or mythic rares in between games during a match, so long as their deck meets the rarity restrictions afterwards.
Strixhaven: School of Mages (STX) and Strixhaven Mystical Archive (STA) are treated as though they are one set for Primordial. Cards from STA may be included in a STX deck at their printed rarity in STA. This rule is also true for The Brother’s War (BRO) and The Brothers’ War Retro Artifacts (BRR). As of this message, the rule is only true of the previously mentioned sets. This rule is not true for sets such as Kaladesh Inventions (MPS) or Amonkhet Invocations (MP2)