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.

way/1484873169 should not be tagged as farmland. It's very clearly used as a paddock for horses/animals, and is not ploughed or mowed.

Noticed you corrected the collection times syntax here, this was automatically entered by streetcomplete. Might be worth opening an issue if the selection dialogue is providing incorrect tags?