Cut props and weapons explanation:
Late reply, but I'm not sure why the weapon attachments, broken road pieces and ammo caches were cut, though my only guess is that they were just lazy
And for the melee weapons, my only guess is that they couldn't be bothered to recolor the blood on the weapons to green

Cut zombie models explanation:
But as for the cut zombie models, because of the game seeming to be directed towards a younger audience, I guess the developer(s) didn't want their audience to be seeing things such as zombie with missing body parts, items jammed into their bodies, ect
though they really could've removed the more "violent" aspects of those models easily given that Synty already had assets that removed those violent aspects of those zombie models, and the fact that those aspects weren't all that violent enough with anyways

And there was a woman zombie with pink hair, though they the model cause the Synty store suggests that the pink-haired zombie was a model of a zombie prostitude
And my only guess for the zombified santa being cut is because they didn't want their directed audience seeing santa as a zombie

In short though: The developers mightve been too lazy to add in the cut models, and some zombie models were just too violent for their audience