Most metropolitan areas. Seattle [1][2] and Minneapolis [3] have both established public policies of not responding to, citing, or arresting for a wide range of offenses.
> I don't know of a single city that has actually reduced their police size to any real sense?
Minneapolis has reduced the number of police officers, through various means, by 30%. [4]
> Additionally, how do we know this isn't just correlation and not causation?
Because we have multiple studies proving a causal relationship. [5][6]
[1] https://www.seattletimes.com/seattle-news/times-watchdog/sea...
[2] https://www.kiro7.com/news/local/seattle-police-bellevue-oth...
[3] https://www.reuters.com/investigates/special-report/usa-poli...
[4] https://www.cbsnews.com/minnesota/news/minnesota-supreme-cou...
[5] https://rss.onlinelibrary.wiley.com/doi/10.1111/rssa.12142