May I respectfully disagree with everything=)

1) Creation of retaining wall is superficial - the basin boundary/wall is one real-world feature and, hence, should be one object. (https://wiki.openstreetmap.org/wiki/One_feature,_one_OSM_element)
Additional information about basin wall that it is a wall and not a slope, IMO, is not sufficient to justify the duplication:
a) that is a responsibility of the person on the ground
b) there is a lot of space to navigate around, so that ability to cross the area is not of high importance

2) I was also surprised, that the basin is tagged as natural, not man-made, but that's what the documentation says! All kinds of water bodies, even constructed, are supposed to be "natural": https://wiki.openstreetmap.org/wiki/Template:Map_Features:basin

+ quote from wiki about "natural" tag: "The natural key is used to describe a wide variety of physical geography, geological and landcover features, including ones that have been modified or created by humans." (https://wiki.openstreetmap.org/wiki/Key:natural)

3) "brownfield" is a new tag for me. From what is in the wiki, it is a tag to describe a landuse, while the basins we are talking about are more like distinct objects/amenities, rather than landuse. Imo, they are objects located within "landuse=park" (albeit unmarked). It is not a lot for re-development, it is ... a disused amenity.

=> I vouch to revert this changeset

Yet, thank you very much for your thoughtful approach!