Replying to your 4 paragraphs one by one:

1. If a DWG member tells you that something is a "golden rule", you should accept the friendly advice instead of trying to argue over it. The DWG are the ones who will ultimately have to make the call whether your edits are constructive or not, so you ought to listen to them.

2. SomeoneElse's point was that you should open a new thread in the forum if you want to continue arguing, instead of doing it here. Exactly so that you get feedback from more community members. That said, the forum is not the only place where community discussion happens. Your changes were discussed on #osm-at IRC/Matrix, which is why several people found them. We were not personally contacted by Negreheb or anybody else, that is what IRC/Matrix is for. And the channel that was used is an official discussion venue for OSM in Austria.

3. We are not Negreheb's or whoever's "friends" (see above), but community members from #osm-at IRC/Matrix. And the facts are that this underground rainwater canal (I would not call it a "sewer" since it does not go into the sewage system, but directly into the Liesing brook) is underground and as such needs a tag that specifies that.

4. What a "tunnel" is for you, or even for cave explorers in general, is not relevant here. For OSM, what matters is the definition of the tunnel=culvert OSM tag: https://wiki.openstreetmap.org/wiki/Tag:tunnel%3Dculvert which states it is "a device used to channel water" and "for a stream/drain/ditch passing under a road". In other words, this tag is intended exactly for the kind of artificial underground canal we have here, according to your own description. Hence, it is not constructive to repeatedly delete this accurate tag.