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These "3D Buildings" are actually other map elements misused in way not to resemble reality but to render in default map display as a 3D image of a building. They are actually made with barriers, buildings, and landuses. Also some sections of roads were not mapped because they were "behind" the "buildings".
Yes, this was actually mapped, it is not a theoretical example.
The canonical example of what not to do. (The accelerator rings were incorrectly tagged as roads so they would render on the map in default view).
Using powerlines to draw antenna map symbol, another example of what not to do
Screen shot of a sports field with an American football end-zone tagged as landuse=commercial to get a pink fill and each letter tagged as natural=sand to get a yellow fill.
An improper mapping of non-existent landuse=commercial and natural=sand areas to paint lettering on a sports field. This is improper mapping because there are not actually commercial or sand areas in these locations, the tags were just chosen for their fill colors in a particular renderer.

The phrase "Tagging for the renderer" and especially "Don't tag for the renderer" has a long history within OSM. More broadly it is sometimes phrased as "Don't map for the renderer" since it's not just about tagging. In any case the phrase is often misunderstood. The meaning of it is closer to the following:

Don't deliberately enter data incorrectly for the renderer

but we need to live with the phrase that gets used most often. The basic good practice principle is that you avoid using incorrect tags, or otherwise skewing the data you enter, to make things show up in a specific way on the map rendering.