Digital-First Cards Yes, Digital-Only Cards/Mechanics No
Historic Horizons is a misguided attempt to chase Hearthstone through emulating some of it's worst aspects - using the digital format as an excuse to introduce high variance gimmicks that are as likely to put off new players as well as existing (it's why I don't play Hearthstone, many of the choices to appeal to the digital platform make the game feel non-interactive and extremely luck based). The notion of cards that are 'printed' first in Arena and then make their way into other sets in paper is something that should be looked at instead. Maybe even utilise that aspect to allow for balancing, so that when the card is printed, it's unlikely to cause issues in the formats it's intended for as any problems can be 'tuned' there. But as a player who greatly enjoys playing Historic as a way to learn and enjoy Magic's history, the coming digital mechanics risk changing a format I love into one I dread.