Odor trap for flat roof inlet

Rainwater significantly strains the capacity of sewage treatment plants and is also ecologically completely nonsensical.

Maybe, but it won't get you anywhere. At least not if the city doesn't plan to tear up all the streets and obligate all owners to tear up their properties to separate the wastewater.

The street section was "piped" shortly after the war, as there were certainly other concerns than the separation of gray and rainwater. But the hint is good, thank you for that, then I will already lay a second pipe in the yard when I repave.

But back to my actual problem: Is there a trap that can be retrofitted into a vertical pipe?

In DN 100 there are so-called fall pipe odor traps. They contain a spring flap that opens when water is present and otherwise closes (i.e.: enter the search term "Fallrohr geruchsverschluss" in an internet search engine of your choice).