All the cards in the Unfinity (Un-set) are black border. Some have regular round/oval stamps, some have an acorn stamp and some have no stamp.
What do the different stamps means?
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The "security stamp" at the bottom of the card replaces the silver border:
If you see a standard oval security stamp, or no security stamp at all (for commons/uncommons), that card is Eternal-legal like any normal black-border card.
If the security stamp is shaped like an acorn, it's a goofy/experimental Un- card.
(Even common Un- cards should have the acorn. Look at Gobsmacked, for example.)
That does mean that a bunch of Unfinity cards are Eternal-legal and you can play them in Legacy, &c. Including a bunch of cards that give stickers, which makes Legacy a sticker format now.
Unfortunately, mistakes were made in production!
A bunch of Unfinity cards were misprinted, so actually you have to look up the card on Gatherer or Scryfall to know whether it should have an acorn or oval stamp if there's any question about set legality.