(answering some of the questions above)

> Are you sure you got your forum link right?  
Yes, I deliberately linked to the Austrian forum.  The international tagging forum is https://community.openstreetmap.org/c/help-and-support/tagging/71 ; I suspect you'd receive even more pushback there.

What you seem to be saying is that it is an "underground drain that is somehow not a tunnel".  Obviously I've never been to the location in question so can't add my own thoughts as to how to tag this particular example, but tunnel tags are regularly used with the underground portions of drains and ditches.

