IIRC, carriers not updating their customers to the most recent Android build has been a big problem.

Not really Google's fault, but perhaps Android isn't a great example.

That causes issues for being able to use the latest features, which is somewhat of a problem, but should not affect backwards compatibility. It means you have to possibly be aware of the OS features you use in your apps, but you don't have to worry about updating them due to some API being deprecated.

It's not necessarily a question of backwards compatibility. Just being forced/pestered to update constantly is an anti-feature.