Well, https://wiki.openstreetmap.org/wiki/Tag:waterway=waterfall says "A place where water flows over a vertical drop or a series of drops in the course of a stream or river." (this definition apparently copied from Wikipedia), which is all fine until we get to "stream or river". Wikipedia has an article "Artificial waterfall" but there's nothing in OSM wiki that I could find about that.

Personally I would consider a fountain to be something that shoots water up, or bubbles it up - not a feature where water is falling down. The fountain here is IMO best kept separate. But I should note that artificial waterfall at Singapore airport is mapped as a fountain https://osm.org/way/720156415

We could add artificial=yes to the waterfall here, but that smells a bit of https://wiki.openstreetmap.org/wiki/Trolltag. Another option could be changing tag to artificial:waterway=waterfall, or man_made=waterfall - then it's going to stay in database for editors to see, but won't render on the main maps (neither of these tags are used at all so far).
