Referring to Minix was not my intent. A short overview of the issue is here. As you apparently know Intel chips after 2008 use Minix for the management engine but I’m not sure to what extent Minix itself is a factor the vulns. The problem is the mere existence of an attack surface hard-wired into processors that can be externally exploited when the purpose of the ME is useless to non-corporate users. Bugs have been discovered that enable attackers to install malicious firmware¹. AMD’s PSP is also a problem and I don’t know if the PSP OS has been revealed. PSP is composed of an ARM processor with Trustzone, but I don’t know what OS Trustzone uses.

I just realized I forgot AMD PSP did not hit until 2013, so I guess there must be a lot of 64bit spychip-free boards out there made from 2008—2013.

1: ⚠ that link is enshitified with autoplay so I suggest using Lynx to access it.