Currently the proxy related contracts exist both in openzeppelin-contracts and openzeppelin-contracts-upgradeable repo, this will cause confusion to users which to use.
And I think the one in openzeppelin-contracts-upgradeable repo is the latest version, for example the UUPSUpgradeable in openzeppelin-contracts doesn't inherit Initializable but the one in openzeppelin-contracts-upgradeable repo does.
IMHO I don't think there're real scenarios where proxy is NOT used for upgrading purpose, so maybe removing proxy related contracts from openzeppelin-contracts repo is the way to go.