“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.
Note: depending on the specific motherboard it could be:
- Windows Command-line (some Surfaces)
- a simple switch on the BIOS configuration (usually on desktop motherboards which are sold as a part)
- set first a password to the setup and then switching it off (notably some Acer laptops)
- removing a screw physically, setting a setup password and switching it off (some Acer laptops probably copied from ChromeOS SKUs)
- Setting secure boot to custom and then clearing the key database (usually business computers)
- (Technically incorrect) setting it to BIOS/CSM mode
- Shout to heavens because that OEM has somehow hard-baked secure boot on the system and does not have CSM options (company shall remain unnamed but the name ends in L)
The new M series? Probably Apple themselves will allow booting other OSes, but I would not count on any official support in getting stuff running.