I own a Mac Book Pro 15" 2018 i9 and a MacMini 2018 i7, booth are equipped with an T2 security chip. It is not possible, even when all Security is disabled and external Boot enabled, to install and run ubuntu (not using the internal nvme ssd, not on an external thunderbolt 3 m2 nvme ssd, not on an external usb-c ssd). The only way to "play" with Linux is using a VM. Does Apple fear Linux aka. Android that much ? or is their Team not sufficient enough to build / program a computer to allow us to run Linux.