Not excusing Rowlings general behavior but a thing about the toilet habits of the past is that it does actually check out in real history.

It did not matter if it was the palace of Versailles or even the church. Men often just relieved themselves in whatever corner they saw fit.

Women were more discreet by relieving themselves under their ball gowns where they stood, then walk away like nothing happened.

Chamber pots did already exist, Versailles did provide them, people just didn’t bother.

The main difference with Rowlings version is that wizards did clean up after themselves magically while muggles used servants.

Except if you didn't live in a class that had servants, the market place was known to accumulate huge amounts of waste as the day went on. For peasants the toilet was “outside”