The PSP toggle is referred to in many forums and git bugs from ~2020-23 with some comments that it only disables higher level access, and only post UEFI, you can see the PSP toggle in a bios config screenshot in the wikipedia article AMD Platform Security Processor - Wikipedia
but this is interesting indeed!
and this well cited look into the difference between thr PSP and the ME plenty of great links off(Purism mention Woot!).
So I don’t love the PSP but it seems to just be the ARM trust zone hardware/software with some extensions, but this requires more study. Execution of onboard dirty tricks loaded via ‘evil maid or supply chain hacking’ seems to be possible even possibly network loadable if picked up via website or processed snippet in an email attachment or other network traffic considering it has DRM access but has no net stack unlike the IME Chapter 4 Basics: TrustZone-M | Embedded Systems Security and TrustZone I seems to mostly be about securely fencing VMs in safe sandboxes in server farms and has little use for consumers beyond perhaps DRM’ed media.
so that takes us back over to our Librem 5s and other ARM CPU gadgets, same deal as AMD it seems….