I'm pretty sure Thrawn's troops (except the one guy with a metal face) were always undead. Bringing back the dead is kind of the Nightsister's thing, and in the group shots, you can see every trooper has ribbons of their clothing wrapped around their uniforms. Whatever caused them to be nigh-unstoppable zombies is something extra, and presumably involved some sacrifice on their part. Savage Opress was undead in the first sense in the Clone Wars cartoon, and presumably in this show so was Marrock.
ANY of this should have been established on the show.
Also, this makes the second zombified badass Wes Chatham has played on a streaming show, all though neither one actually shows you as much.
I thought the ribbons around their armor were like duct tape because they didn't have enough replacement parts so far from home and this was an indication everything was salvaged and repaired and restored as best they could.
Perhaps I'm wrong and the 'death troopers' are not all undead. If Wes Chatham is playing the guy I think he is, then he's the only one who's probably NOT undead, though. Taping armor together is a strange idea, but not a ridiculous assumption based on these writers.