Hello everyone,
with the Un-set Unfinity scheduled for October 2022 we are looking forward to a new card type. The so called Acorn cards will replace what silver bordered cards did in the past. Both types of cards are not intended for tournament play. This is why silver bordered cards are not legal in Archon and why we update now our rules that cards with an acorn security stamp will be also not legal. You can find more about acorn cards here: https://mtg.fandom.com/wiki/Acorn