"Flophouses" or SROs used to provide affordable housing for young people new to a city, single people, workers, etc but they depend on density and transit to be cheap. They were largely made illegal in the mid-20th century. Land use/zoning laws are why we've built nothing but car-dependent suburbs for the last 50 years.
SRO's are for single people, but they're a form of shared housing, so I think the point stands.
They are still around but less common. Some progressive cities like Austin calls them a "Boarding House"[0] I'm not sure if there is a unit cap where something like your historical tenement housing could happen like in Manhattan. It's the same idea just a different format.
It has not been forever true that households have had two incomes. Now it is common.
I meant it's been true forever that people with partners generally have a financial advantage over single people. (Even if they're not in a two-income household.)
What has changed is that dual income became the norm, and is no longer an advantage, but mandatory to compete on the housing market.
When single income was the norm, the norm was to put a family of 6 in an 800-1200 sq ft residence. That's still roughly equivalently affordable today.
Well, only because one of the household did a huge amount of unpaid labor. A lot of which now has been replaced with paid labor (child care, restaurants, house cleaners, and so on).