I know, I don't have any proof, but it seems to me that when you go heal your adc with relic playing as thresh, it don't "execute" the minion, even if they under the amount of hp needed, stated in the description of relic. Any reds can confirm this is normal or anyone else notice this too? 
#ACTUALLY READ THE ITEM ABILITY.
Only melee champions can execute minions with Relic Shield.
Thresh is ranged.
They should just change relic so any hit would execute.
You can proc it with thresh you just have to be close enough for him to not fling his hook, just swing it
I facepalm every time I see one of these threads
Then why does rito suggest it on thresh? Thresh also heals with it though. That's the bs about it.
Read the tooltip.
http://leagueoflegends.wikia.com/wiki/Relic_Shield
Unique – Spoils of War: Melee autoattacks execute minions below 200 health. Killing a minion by any means heals you and the nearest allied champion for 40 health and grants them kill gold.
Thresh is a ranged champion so he doesn't proc the melee-only execute.
It is still recommended on him since:
If you aren't able to use it, nothing is stopping you from getting Ancient Coin or Spellthief's Edge.
