My assumption is that undervolting was disabled because no other mitigation for the Plundervolt vulnerability could be identified that could be implemented for already-shipping processors (i.e. implementable only through microcode changes). Presuming that I am correct, this means that, if it is possible to implement a better mitigation in future processors, it would be possible to reenable undervolting. That's a big 'if', however; I certainly wouldn't bet money on this happening.
...S
