killed :/

Azir got killed by Xerath Kalista got killed by Hecarim

I think those are the only ones.

Original Gangplank was "killed" and people could only play him as Spooky Gangplank for a while. Then he rebirthed as the current Gangplank.

Mordekaiser was killed over and over again, not necessarily by champions though. Gangplank was supposedly dead after MF blew up his ship (she still thinks he is) but obviously they can't let him stay dead. That would be like they removed your favourite champion from the game, so every in-lore death will either be fake like Gangplanks, undone by magic like Azirs or simply undeath like the Shadow Isles champions.

Technically Xerath killed Azir but that was during the Ascension mumbojumbo.