Thresh have a hidden/secret passive called "Soul Glutton" that it appears after eating more than 255 souls in a game. (as you can see here)
But when i was playing with him (testing the new skin), the buff/debuff hud looked like this
Looks like the "Soul Glutton" passive doesnt appear when you got more than 255 souls.
1. Take Thresh
2. Play him
3. Collect more than 255 souls
4. See results (no "Soul Glutton" passive appears)
Also, if someone knows how to put screenshots properly in a thread, answer me please, Thanks you