It felt weird creating another function that would only split this key up, that's very tightly coupled with getDevicesByPlatform. But I can do that if it's better

It may look obvious for you but it makes it hard for new developer working on this code to understand what is going on. Additionally any change in getDevicesByPlatform will require you to change all places where it is used. It is a flaw of JavaScript that we can't define custom hash functions so that any object can a used as a key inside map. What I suggest that we do here is that:

1. Introduce type (not necessarily exported) that covers codeVersion and stateVersion.
2. Function that converts it to key according to your idea with "${}|${}".
3. Function that converts string back to codeVersion and stateVersion pair and uses regex with invariant to ensure we can't pass anything there.

Alternatively - It looks like the output of getDevicesByPlatform is never actually used as a random-access hash map. It is always iterated on. I think we could easily and safely refactor it to return

Array<{
    +codeVersion: ?number,
    +stateVersion: ?number,
    +devices: $ReadOnlyArray<NotificationTargetDevice>?

}>

We can use still the trick with merging to a string and splitting it back to ensure uniqueness but it will be used only inside its implementation and won't hurt code quality in other places in this file.