What/how should old (no longer there) river courses be tagged? Should they be otherwise normally tagged but with abandoned=yes added, or perhaps historic:waterway=river for example?
I would imagine a lot of people would just say don’t tag them because it is not necessary, but I would say that perhaps it is, or at least it is interesting. Many other things can be tagged once they no longer exist, such as historic railways, roads, buildings etc so why not waterways? Often they are of significant historical importance in the same way that the aforementioned things are. Personally I am quite interested, as I’m sure many others are, with OSM’s capacity as a tool for historical mapping and, hopefully one day, research, which is why I assume historical tags exist to a greater or lesser extent for things that no longer visibly exist. Of course it would not be necessary for historic watercourses to be rendered in the standard renderers, but I would say it is valuable to record them nonetheless.