She can be reborn, but iirc phoenixes need to recover their strength in between. If she was killed twice in a row, Anivia could cease to exist.
has been bound by Targonian magic to do their bidding. It's a safe bet that they could kill him, given the appropriate opportunity. Aurelion Sol also regards the Void as a threat, so it's likely that he could be killed the Void itself.
Vayne saw Evelynn in her lore, and that's what inspired her to become a night hunter. Stands to reason that Evelynn has a physical form and could be killed, given the right circumstances.

Illaoi was able to hurt/turn aside Hecarim in Shadow and Fortune, so I'd say it's a safe bet she could kill the creatures of the Shadow Isles under the right circumstances. Demacian sunsteel is also effective against the living dead and the wraiths of the Isles.
falls into this category too, given that he's a zombie as well.
Was killed multiple times in his own lore. If his liches were slain, and then he was slain and his skeleton destroyed, Mordekaiser could be unmade.
Afaik nothing in Maokai's lore says he's unkillable - in fact, I believe he almost dies in his short story while protecting the flower from the wraiths of the Shadow Isles. Fire and axes could definitely hurt him.
Her sisters were both Iceborn and perished after the rebellion, so it again stands to reason that Lissandra could be killed. Clearly she can't die of old age though.
I'll give you
and
, as we haven't really seen them interact with the world in a tangible way. Doesn't mean they're unkillable, but we don't have any evidence to disprove that.