According to several sources, it seems most current AM4 BIOSes enable the ability to enable/disable this feature as part of their AGESA updates. [strike]I have 2 AM4 systems, I'll check their UEFIs and let you know.[/strike] Seems these sources may not be correct, or they don't understand this issue and are only inferring something not accurate.
Weep? A tad bit over dramatic, I say. I understand the technical issue, I was wondering if you do. People often read something and fail to understand its intent, resulting in irrational decisions based on FUD.
Neither my Asus Crosshair VI Hero based rig, nor my Asus X370-Pro Prime based rig has a setting to disable PSP in the UEFI settings. I updated to the most recent BIOS updates available for both.
As there are no known threats to attack PSP (per those with this concern), I wouldn't overthink this. Of course, an exploit could be developed. This concern is also a possibility for Intel based rigs (IME).
If this is a major concern, you can always go the Power9 route:
https://raptorcs.com/