If within the 150 units (or whatever range it is for him to switch to the melee animation with the sickle) of the minion he hits with a basic attack, he will proc the 
passive execute damage.
Is this really too much to ask for?
EndlessSorcerer3 votesIt really isn't that hard to last-hit using Flay's passive.
I don't see why they should bother making a specific exception for Thresh.
JavelinJoe1 votesI would rather use the passive on the enemy in fights and for poke/harass.
EndlessSorcerer1 votesFlay's passive will be consumed with or without Thresh being able to execute using Relic Shield.
Try again.
JavelinJoe1 votesYes it will.
But, with this change, I do not have to wait to use the higher stored damage just to ensure I can kill said minion. I can simply auto any time, even if I just spent the passive on an enemy champion, and not have to waste time and miss potential poke/harass opportunities waiting for it to recharge to hit a minion, wait for even more time, than attack the enemy again.
EndlessSorcerer2 votesIf you know how to last-hit properly, the bonus damage isn't necessary anyways.
It is a useful crutch, but by not means necessary.
JavelinJoe1 votesTell that to useless ADCs who love to last hit every minion even if you spam chat and ping that you are taking that minion.
EndlessSorcerer1 votesI've never really had that issue.
I just ping whatever minion I'm planning to kill before attacking it and the ADC will leave it for me.
JavelinJoe1 votesYeah, any ADC who I support with as Thresh who is also under Gold 5 can't seem to get this.
EndlessSorcerer1 votesAh, that might be the issue.
I primarily play with Platinum and Diamond players, so there are considerably fewer issues with communication.
Are you using AD marks? They tend to help both last-hitting and harass.
JavelinJoe1 votesIf I remember to switch to them, which I usually do. Sometimes I run a full tank page though if we really need just a wall of meat but usually I have AD runes.