Were seeing the same thing happening again with Spirit of the Labyrinth
'Legend' was originally just a creature type in these older cards, but in the future they would become essentially a supertype -- an extra adjective that tells you that this card falls under 'Legend rules', which means that only one copy of a card with the same name can exist on the board at any time (later changed to only one copy existing under the control of a player)
As the Pokmon universe expanded, so too did the TCG, with new sets introducing fresh Pokmon, abilities, and gameplay elements
The speed boost to mill opponents is likely to help opponents that want cards in their graveyard unless you're turbo charging your other options to churn through opposing libraries alongside this
Copying spells has well-defined rules in paper, so even though there's a tracking issue with playing a card like this in person, it's not new territory for physical Magic cards