They can be configured to boot other operating systems, see https://support.apple.com/guide/mac-help/macos-recovery-a-ma....
“Review the following security options:
Full Security: Ensures that only your current OS, or signed operating system software currently trusted by Apple, can run. This mode requires a network connection at software installation time.
Reduced Security: Allows any version of signed operating system software ever trusted by Apple to run.”
⇒ it appears ‘other’ still requires OSes signed by Apple, but it allows you to run an OS even if Apple has no longer trusts it. Or am I misreading that?
So, there might be Apple-signed Linuxes, but I wouldn’t hold my breadth for that.
Eh, I read reduced mode as basically how secure boot works on hardware with Microsoft-only keys in which MS signs a shim and it automatically blesses desendants (technically the OEM is not restricted to put other keys on the key database, but aside from business computers it seems that most machines have only Microsoft and OEM keys). Of course, Microsoft has a stronger incentive to sign these (or else they will be accused of antitrust violations), so Apple might not do that.