I've run into this a few times with Apple. It's not that the upgrade is explicitly not allowed by policy. It's just that things are broken in ways which prevent you upgrading/using the device properly. I don't know if it's just lack of care on Apple's part, incompetence, honest mistakes or a stealth method of forcing an upgrade. In any case, hardware that seemed slow and outdated with Apple's software ecosystem now runs perfectly with Ubuntu. ¯\_(ツ)_/¯