I mean for example this building, where it is only partially covered by building parts now way/921508463. In some 3D renderers, the left hand side of the building will not be rendered, because it is not covered by a building part.

I think you are misinterpreting the part of the wiki that you quoted - it says that you should use the tag building:part=yes for building parts that have the same function as the surrounding building, so in this case building=apartments and the parts within are tagged building:part=yes. It does not say that if the attributes of a part of the building are the same as the surrounding building that no building part is necessary.

The wiki page should make it clear that the whole footprint of a building should be covered with building parts, according to the Simple 3D Buildings schema. I will try to get someone to update the German page. The English page says this:

"Common mistakes:
Missing parts: If a building's parts cover only part of the building outline, the remainder of the building may not be visible in a standard-compliant 3D renderer."

Hopefully that is clear. Fröhliche Mappierung!

Hey, it's not correct to delete these building parts here. As part of the simple 3D buildings schema, a building polygon needs to be completely covered by building parts, even if those building parts have the same building:levels tagged as the surrounding building. So for example way/946524532 wasn't unecessary or duplicative, but is required for representing the building correctly in 3D. You should also not remove tags like roof:shape and building:colour from building parts where they are the same as the surrounding building polygon's tags - while renderers like F4 assume building parts have the same properties as their surrounding building, it is actually helpful to explicitly place these tags on each building part.

osm.wiki/Simple_3D_Buildings - "The entire building outline should be filled with building:part=* areas, tagged with their respective height and other attributes."

Thanks.