Arcane Signet is technically Modern-legal but sees its real popularity in Commander and casual multi-format collections. Two-mana rocks tend to be too slow or fragile for most competitive Modern decks compared to land-based ramp and cheap spells. That said, players who port their Commander decks into Modern-style casual pods often keep Arcane Signet as familiar ramp. Its ubiquity in preconstructed products helps it show up in many card pools even if it is not a Modern staple.