Digital-only cards have a long history of Magic with the first 12 showing up on Shandalar, a 90s video game version of Magic where players adventure along a map and dungeons battling AI opponents while building the strongest deck they can
The trick is that you need infinite mill combos to mill 100 cards
I believe it was rightly banned in old Extended and should remain so in Legacy.
I think it will be included in ramp decks that secure lands through milling, but it is troubling that Chapter 1 provides almost no advantage
Your current mission might be unearthing that elusive shiny Charizard, tracking down a specific Special Illustration Rare to complete a master set, or securing a deck-defining staple card to crush your next regional championship
Cheap key cards: Goblin Warchief, Skirk Prospector, Impact Tremors, Goblin Chieftain, Krenkos Command, Shamanic Revelation