They did in fact say “It’s a feature not a bug” and left it in game :(

Zombie Heavy is no longer immune to Critical Hits

Finally. There were too many times where I, playing demoknight charge in an unsuspecting Zombie with perfect crit timing, only for it to be a heavy and fuck my shit up; even with the shield bash. -54 to a fucking tank with 450 health was ridiculous. Glad they nerfed it. --260 to a 525hp heavy is much more balanced.