Champions with true damage:
Ahri (Q)
Amumu (passive)
Aurelion Sol (E execute)
Bel’Veth ®
Camille (Q)
Cho’Gath ®
Darius ®
Draven ®
Fiora (passive + R)
Gangplank (passive + R)
Garen ®
Gwen (Q)
Kog’Maw (passive)
K’Sante (passive)
Lillia (Q)
Master Yi (E)
Olaf (E)
Pyke ®
Rek’Sai (E)
Sett (W)
Syndra (W with fully stacked passive + R execute)
Twitch (passive)
Urgot (R execute)
Vayne (Q)
Vel’Koz (passive)
Yone (E)
Zoe (E)
I’m not a huge fan of true damage. I think true damage should be reserved for Ignite, making it a unique damage type. Everything else should simply have bonus armor/magic resist penetration if they want to amp up the damage. But leave true damage out of champion kits and items.