Plundervolt is a similar class of Undervolting attacks like CLKScrew and VoltJockey executed on SGX enclaves.
This is because the undervolting interface is only accessible with root privileges in the untrusted operating system - it would not make sense to attack software with undervolting when you're already root, apart from the case of SGX (which should protect against a root attacker).
No. The undervolting interface is accessible from software, so if a remote attacker can become root in the untrusted OS, she can also mount the Plundervolt attack.
If you do use SGX: Intel has released a microcode update that - together with a BIOS update - allows disabling of the undervolting interface.
However, Plundervolt and CLKScrew/VoltJockey are similar in that they use a privileged power/clock management feature to inject faults into a trusted execution environment.
Plundervolt achieves the complementary operation, namely changing values in SGX-protected memory (i.e. attacks the integrity).