Illaoi un "E" preciated. [Gameplay]
Basically this: Illaoi uses E, connects, pulls spirit out, other teammates kill the target before you can damage the spirit. You receive no assist. Since the skill shot connected, you should receive an assist regardless of if your hyper carry fed teammates annihilate the target before you are able to lay into the spirit.
Counter point, thresh walking down mid lane throws a lantern on someone involved in a fight somewhere a tower deep yet now past own tower and no longer contributing to the fight, target dies way down midlane without thresh ever being there. Assist received. Good job thresh!