This is more of a theoretical discussion.

In order to properly vent a fixture, the trap arm must be less than a certain maximum length from the trap outlet to the sanitary tee where it ties into a stack or to the wye in the case of the wet vent. My understanding is this is determined by the combination of slope and distance to ensure the air along that path is not going to be cut off during normal typical flow conditions.

For example, for a 1/4" per foot slope, a 2" pipe longer than 8' will have the vent inlet connection flowing "full" and create a problem.

But if you reduce the slope to 1/8" per foot slope you could actually have a length as long as 16' before running into trouble.

Theoretically, if you lay your trap arm at zero slope - completely flat - you can go on forever, without any issue theoretically.

Yet the code requires 1/4" per foot everywhere right?

Does any city allow a plumber to reduce the slope to less than 1/4" per foot or go totally flat to squeeze a few more feet beyond the maximum length? If not, why not? Even if you make it flat water will still flow and drain. Is the 1/4" per foot slope to scour the pipe in the event it has solid such as kitchen waste from disposer or toilet waste?

i was taught that 1/4 inch slope has the water travel at 2 feet per second which is ideal for washing things along and scouring the sides as well. this is all calculated on minimum pipe size. obviously a lav drain will not scour a 4 inch main but it will on a 1 1/2.. (to an extent)..
if i want to extend a trap arm beyond the maximum allowed, i increase the pipe size.

The 1/4"/ft slope is dependent on the pipe size. 4" and larger can use a 1/8" slope, and larger main sewers can get by with 1/16". It IS to maintain a certain velocity to maintain scouring action. NO ONE would allow zero slope and you can only use less than the required slope if you have a mechanical engineer design the system and stamp it that it will work properly.