New Apple T2-enabled hardware no longer allows Linux to be used.
https://fossbytes.com/apple-t2-security-chip-blocking-linux-booting-new-macs/
https://www.theregister.co.uk/2018/11/06/apple_mac_linux_woes/
https://www.trustedreviews.com/news/apple-t2-chip-linux-mac-problems-3616180