I'd say it's the same rules as any other (swimmable) river in France, from the big to the small. Maybe it'd make more sense just for smaller rivers, but where do you draw the line? Like it's easier and simpler to just use the same set of rules

Maybe that was the initial thought? It’s a river, a handful of homes emptying sewage there is meaningless. And centuries later, here we are.

This is true, but shortsighted. The overall Paris-Plage plan is a resounding success. Re-opening the Seine to swimming is likely to be just as popular.