As in "lacks running water," meaning you have to dispose of a building's worth of piss and shit at street level multiple times per day. And if it's a residential building, that means no water for bathing, cooking, drinking, etc. (You could ship in thousands of gallon jugs per day, but that's a logistical nightmare.)

Like a world without elevators, urban life without plumbing very quickly becomes unsustainable in buildings above a certain size.

The piss and shit problem is real, but not nearly as bad as you're imagining; I've written a bit based on my extensive shit-related experience and shit-related human history in https://news.ycombinator.com/item?id=45908441.

Burning Man recommends 6 liters of water per day per person for bathing, cooking, drinking, etc.: https://burningman.org/event/preparation/playa-living/water/ But that can generally be cut in half when you're anywhere other than one of the world's driest deserts in the middle of summer. This means you need to bring in your own weight in water roughly once a month; if there's no elevator, and you can safely lift 25% of your own weight, you can lug it up the stairs once a week. If there's an elevator, you can probably bring the water up the elevator every month or two.

About a third of that water has to eventually go back down in the form of piss, which is not a major problem if you have sealable plastic bottles to store it in. There's always the risk of an unpleasant accident with that approach, of course, but that's rare.

So I don't think there's ever a building size where a lack of indoor plumbing makes urban life unsustainable. If you're strong enough to walk up and down the stairs every day, you're strong enough to carry water up the stairs once a week. If there's an elevator that you can ride carrying two children, you can also ride it carrying water, once a month. Throughout history, and today in poor rural areas, most people have always had to carry their water much farther than that.