Please consider the risks of the following vulnerability before deciding whether or not to undervolt: https://plundervolt.com/
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I used https://github.com/datasone/setup_var.efi to modify the UEFI variables. The README has all the info you'd need. It turns out that both a BIOS and microcode update is required to kill off this feature, and you could just configure the BIOS to not lock it.