Wasn’t Indus Valley around 5800BC?
Their neolithic beginnings were in the 8th m BC. Their city-building didn’t get going until the 4th m BC, and they didn’t immediately build plumbing — they started out in small villages, and like a lot of riverine civs, went with massive agriculture, and then needed recordkeeping to track who farmed what, and how much irrigation water was needed; urban living was due to more and more descendants building out from the village cores. That kind of crowding led to the usual learning from mistakes, and sewage management.
The first of what we’d consider somewhat modern plumbing isn’t (as is usually claimed) Knossos on Crete, but the city of Rome. They used similar methods (a buried channel with running water to flush it out) of sewage handling as the Harappans had. Other civilizations handled it differently, the most common method may have been, “go” in a container, and every day or some other interval, take it to the designated area a little downstream from the water supply.