Does Heimerdinger's ability only copy from cards currently in play, or can it copy from cards in other zones like the trash?

Ruling: Heimerdinger can copy exhaust abilities from cards you or your ally controls that are currently in play (units and gear) or in the Legend Zone (legends). It cannot copy abilities from cards in the trash or other zones.

Nuances:

• Cards only apply their effects while in play unless stated otherwise
• Legends are an exception because Heimerdinger specifically references "Friendly Legend" and legends can only exist in the Legend Zone
• If Heimerdinger is alone in play with no other units, gear, or legends with exhaust abilities, it has nothing to copy