Pioneer is the newest MTG format. Like Modern before it, Pioneer was designed as a place for decks to be used after they rotate out of Standard. It includes all main sets since Return to Ravnica.
These cards cannot be used in this format:
Bloodstained Mire · Felidar Guardian · Field Of The Dead · Flooded Strand · Leyline Of Abundance · Nexus Of Fate · Oath Of Nissa · Oko, Thief Of Crowns · Once Upon A Time · Polluted Delta · Smuggler's Copter · Veil Of Summer · Windswept Heath · Wooded Foothills