Is a Firebase UID always 28 characters?

A Firebase developer had this to say on their forums:

I would not rely on a constant length for the UIDs forever. We chose the current value because it provides a big enough ID space while still being under the 36-character limit for Firebase Analytics user properties.

So, in other words, what you're suggesting may or may not fail in the future, so you probably should find a different way of implementing it that's more future-proof.

The uid of type string is used to assign to the newly created user. Must be a string between 1 and 128 characters long, inclusive. If not provided, a random uid will be automatically generated.

https://firebase.google.com/docs/auth/admin/manage-users