That's not true, different api levels have different restrictions and defaults even if you don't change any of your app code, that's why they force you to target a new api level.
For example, one Android 15 change is "Apps that target Android 15 must be the top app or running an audio-related foreground service in order to request audio focus.", or another one "For apps targeting Android 15, the `elegantTextHeight` `TextView` attribute becomes `true` by default".
It's not always a no-op. There's zero chance that storage costs are the concern here, they've long struggled to fix overly permissive APIs and poorly designed legacy APIs on apps that target old api levels.