In the case where a multipolygon is composed of lines with no tags except "source:geometry," let's say we replace this multipolygon with a simple polygon. What do we do with these two lines? If they can remain in the database and provide a source for other contributors, then I don't see any problem with switching to simple polygons.

My main concern is the sources, as long as the sources are present on the objects; I don't care whether we use polygons or multipolygons. Sources in diff comments (when there are any) are nice, but apart from the fact that you have to search the object's history each time, it doesn't allow you to work with cross-referenced or complementary sources!

(POV) However, if I had to argue in favor of multipolygons, in the long term, what would be the storage space saved by having a widespread use of relations listing shared linestrings instead of polygons listing nodes (boundaries work on this principle and it works). If I take the case of the Athénée Royal de Rochefort, it is 4 linestrings listed instead of 24 points. And knowing that by using sources like the PICC, the GRB or recent aerials, we can easily multiply the number of geometry points by 10, I let you imagine the potential savings.