The ERC1967Proxy inherits ERC1967Upgrade which in turn has proxy admin management functions (e.g. _changeAdmin). All of those functions are either private or internal and are never used in the ERC1967Proxy inheriting them.
I guess the functions are reserved for those who want to modify ERC1967Proxy. But why would anyone need it when all the logic (including upgradability) is on the implementation side? Is there a real world example?