Keys are also essential to retaining an element identity, which is structurally more important than performance, because 1) code may have a reference to an element and think that it relates to some data, 2) an element may have an explicit state (like focused, animating, etc).

Keys are a hugely leaking abstraction, but are inevitable when you bridge a declarative immediate mode rendering into a stateful one.

I believe you can specify a key in Svelte as well, but perhaps it's used slightly differently than react.

Yeah it's not strictly necessary, but if you are updating the list (adding/removing especially), Svelte can know how to reuse elements properly if they are keyed. You also need keys for animations to work properly.