I know of single room rentals available in pretty much every major metro in the world. Shared common bathroom and kitchen.
I also know of one in plenty of subletting of multi-bedroom apartments.
I have never heard of enforcement against this. It also doesn’t bring rents down as much as claimed.
I'm not sure if this an actual law but housing listings often imply its forbidden in the neighborhood, they're looking for couples and families with kids.
The landlord believes that their property and their relationships with the neighbours of the property will be less likely to destroyed by letting the property to older/respected/settled down members of society. Common practice in most of UK as well.
Because eventually individual rooms start being rented by families. Next you have four families living in a single-family occupancy location and there is a huge fire hazard. I've seen this happen in NYC growing up, and its super dangerous. I also empathize with the other side -- as a poor person you may have no other option.
It’s not at all clear to me that four families in a single family-intended house is worse than the alternatives. (Building more housing is the long-term solution, of course…)
Therefore regulating housing is quite possible to only make things less safe, as people end up giving up money for healthy food / education / healthcare / dentistry etc to fund the trumped up "enviromental study" "planning and zoning" "code" and other requirements that might not best fit their budget.
In practice I think it's about impossible to enforce. Code enforcement or police would need a warrant to enter, and in most jurisdictions the complaints are public record far enough ahead of time anyone with the slightest bit of foresight would get ahead of it.
In my county sometimes I monitor the local complaints, mostly initially when I was looking at properties because I did not want to live next to a neighbor who likes to be a busybody to the code enforcement. There are a number of properties that just lock their gates whenever a code complaints happen or tell code inspection to kick rocks, by the time they come back with a warrant the situation is faked well enough they can't do anything.
If they have a suspicion and they feel inclined to go after you they'll just go hard enforcing all manner of other shit they don't need to go inside to enforce agains the landlord. It doesn't matter that the things they're trying to enforce may very well be bullshit that couldn't stand in court if challenged, it's cheaper to comply than to fight it.
Code enforcement and other civil and administrative areas of law where the .gov can issue fines on the same order or larger than many criminal penalties while giving the accused none of the rights of criminal trial are a massive, massive, massive, I can't say it enough, massive, end run around constitutional rights.
You aren't in the neighborhoods where this has been in place. But it doesn't mean its not happening.