If I recall correctly, this began in the core set with Garruk, Primal Hunter with a rough five-cost with triple Green, but then +1 to make a 3/3 Beast that's Green or -3 once to draw
If the spell being copied has an X whose value was determined as it was cast (like Debt to the Deathless has), the copy has the same value of X
Basal Thrull is an early example, showing the creation of an original recipe Thrull back in the dark days of Fallen Empires
Standard Ramp in Standard shifts with each set
While treasure tokens were first introduced to Magic: The Gathering way back in 2017s Ixalan block, it was only during the Norse-mythology themed set Kaldheim released earlier this year that their strength started to really catch the eye of deck-builders
Anecdotally, every blue player I know loathes red