Title says it all, I broke Pantheon's shield as Blitzkrank when he tower dove, wasn't able to get a second shot in before he left. He died on the way out, and I got no credit for it. Not QQ'ing, just a bug I noticed, unless it's working as intended.

He blocked your shot. You didn't hit him. You didn't deal damage. Seems obvious to me why you don't get an assist.

It's hard for Riot to pick up things like this when there are soooo many huge bugs in the game. I mean it's pretty obvious how they could overlook this.

Btw, you should change the thread title. There's no way breaking Pantheon's shield cheated you out of a kill, you can't get the last damage in with an action that doesn't deal damage haha...

To get assist credit, you must damage or debuff an opponent. Since you did neither, no assist credit for you.

All have solid points.
Remember it for future occasions;

Hit him twice or use ignite.

It's not a debuff.

The issue here is the way assists are dealt with. If you play a support champ like Soraka and heal a teammate who then kills someone, you don't get an assist either, AFAIK.