From that page:

“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.