Relic Shield is a bit faulty

The execute damage only applies to melee attacks, and Thresh is Ranged. You can proc the gold/heal by last hitting still.

Thresh is counted as a ranged champion. That is why it doesn't proc the relic shield. It would be cool if Thresh could be an exception to that rule.

I figured that out. Thresh, in technical terms, is using a whip. Which in my terms is not a "ranged" weapon.