1) You misinterpret the "One feature, one OSM element" practice. Its main point is that tags that mean the same should not be duplicated in two OSM entities, and it explicitly states that one OSM entity should only contain tags relating to one processable feature. That's just what I was talking all this time: barrier & the feature enclosed by it should not be mapped as one element, there should be a line for the barrier and a (multi)polygon for the area.
Nothing of the above means that there shouldn't be an entity for the border if there is an entity for the area.
And I definitely insist that a wall should be mapped. We are not constrained by any technical means, so we can map as much info as we can describe. There is no such thing as "responsibility of the person on the ground", and no such thing as inherent "importance" of one thing or other. If something exists, it can freely be mapped ;)) [With possible exceptions for military, personal info etc.]
You can never know how the information from OSM will be used. One can generate maps for sport orienteering using exports from OSM, like this: https://iorient.ru/download/Maps/2023/Spring-in-Jrvezh/map.png
In such cases the difference between walls and slopes is of critical importance.
(to be continued)