I dont particular care about removal or not, but

1. you can still void if you want,
2. this info is not hidden exclusively in the structure guidlines but is -of course- well explained in the quest.

the carbon monoxide is of some importance to certain processing chains (iirc titanium?). making them 100% recycled would have a slight balance implications

in titanium processing, the carbon monoxide comes from the chemical reactor step

for ilmenite dust processing (which most of the time gets replaced by chemical bathing the crushed ore), the carbon dust does get used in the ebf, giving back carbon monoxide

other recipes such as cobaltite also gives back sulfur monoxide (or dioxide?) making it possible to loop back the oxygen

Just to make this clear; is your suggestion to:

• Remove the ability to place output hatches in the top layer, and output all fluids using the current mechanics from the bottom layer;
• As above, but also remove the pollution gas output scaling with muffler efficiency (always output 100% of the recipe);
• Remove pollution gas outputs from recipes altogether?