O tempora, o mores.
Microwaves were discovered already in the 1800s with the legendary polymath Chandra Bose being the first to produce mm waves. When it became practical microwaves started to be used for communications and later in other useful things like ovens.
Imagining and using microwaves as a weapon of repression that leaves no obvious evidence of its use (besides the victims of course) requires some genuinely meritocratic mindset and establishment.
If a psychopathic regime with an army of sociopathic enforcers came up with this what could possibly go wrong. Well nothing in their minds obviously. Rule-by-law and all that.
While it may sound harsh, the people are ultimately responsible for the actions they allow to be taken in their name. Even if trying to hold the regime accountable will result in the people getting microwaved to early retirement or kingdom come.