Consider contract @openzeppelin5/contracts/proxy/utils/Initializable.sol where INITIALIZABLE_STORAGE constant is defined as private. It makes sense, Initializable is base contract.
Now consider AppUpgradeableContract.sol which inherits from Initializable and keeps application specific logic. It defines own CONTRACT_STORAGE constant
Now to my question: is it important to keep CONTRACT_STORAGE private? From my perspective it can be made public IF AppUpgradeableContract.sol will be inherited ONLY by a newer version of application
I think that makes sense (or a similar thing can be said but for the storage slot getter which uses that constant). Normally you'd want it to be private, otherwise it would be like exposing private variables for inheritance.