It would be nice if pollution could leak out from the underground. I mean, like with the Factorissimo2 mod where pollution leaks out from the active factories.

I'm not sure that with the current restrictions on what can be placed underground that there would be any pollution producing entities that would be of concern. The author seems to mainly see the mod as a way to have actual underground belts. I suspect the power thing is so you can use inserts and maybe have underground buffers and the like.

When/if I get around to implementing underground mining and oil drilling I may consider tinkering with pollution, but right now most entities allowed underground produce negligible pollution.

I had the idea for creating pollution pumps which move pollution underground (to mimic Carbon Capture Storage) but that ends up being functionally the same as the Air Filters mod since there are no adverse effects to underground pollution (so far).