devtrackers.gg
To actually answer your questions, OP, the barrel armor was something the art director slapped together in an hour and put in as a joke. They don't think it's good enough to be an actual armor set, which is why it's not in the game. But it's been seven years and people still can't let it go.
The mutator that reduces light was deemed unfit because light management is too important for gameplay to be modified like that. It's also one of the reasons why flare upgrades were removed - they wanted a baseline of how flares perform.
Right on the money.