San Francisco certainly did used to enforce quality of life laws, and they certainly have intentionally relaxed that enforcement: “based on the sit/lie ordinance [prohibiting people from sitting or lying on the sidewalk], infractions peaked at 1,011 in 2013, but since that time they have steadily declined, falling to 114 in 2017. ... SFPD's current policy focuses on steering homeless people to shelters, rather than arresting them” ( https://en.wikipedia.org/wiki/Sit-lie_ordinance#San_Francisc... ).

Incidentally, the city believes there are roughly the same number of homeless people today as there were back when they enforced quality of life laws: “The sense that homelessness itself is exploding in San Francisco is not reflected in the city’s numbers. According to the Department of Homelessness and Supportive Housing, the city’s homeless population has remained essentially flat in recent years. The agency counted 7,499 people experiencing homelessness in 2017, about 2 percent higher than 2013 levels, and slightly lower than in 2015” ( https://www.citylab.com/equity/2018/08/san-franciscos-sidewa... ). They claim that back then, there wasn’t crap on the street because people were crapping in vacant buildings. Now, because of the building boom, there aren’t many vacant buildings.

I’m not saying that hassling people for crapping on the street is the right approach. I understand why San Franciscans by and large rejected it. But it absolutely can be done, because that is what they used to do.