I AM the writers room wrote: Hopefully we won't have to wait until season 12 to realise that the unknown characters are dead would like to see Amber, Tanya, Frankie Sherry and Dwight. The rest are just cannon fodder, I think killing off Regina and Jed was the best decision to get rid of old villains and introduce the new ones.
Guess it has to do with Carol's storyline - showing how dark she can get. They could have killed off Jed and Regina in 9x05 but they wanted to show Carol burning them all last night (call back to when she burned those Saviors at the slaughterhouse in Season 6?)
As for Mark, he's most likely dead. Sort of like Huck and the others. But if people strongly feel that he should be unknown instead, that is fine. If they show Amber again somewhere, then there is a possibility that Mark is alive too.