Basically oldest microcode available for 906ED right now is AA rev (20181129)
but is it possible to boot without microcode at all, like it was in haswell skylake etc? because this i think already has spectre patches that tank performance.
I expect it would not boot at all.
Coffee Lake Revision R0 CPU’s (CPUID 906ED) have built-in (in-silicon) hardware mitigation against Spectre v2 (Branch Target Injection) so microcode version becomes irrelevant.
Note that spectre v2 is mitigated in Windows with software as well.
You can use mbk1969’s tool to disable spectre and meltdown mitigations via the registry:
thank you for your answer, will check out the tool.