Here is what would happen. People would mark the areas they feel unsafe in and that alone will cause problems. Because the reality is those areas would “disproportionately” be areas of high crime and/or economically stressed areas. And some “savior” group will decide that they are discriminating against poor people and/or minorities.
But since you can’t go after “the Internet” the company, or companies, supporting the app(s) would be targeted. These people who only wanted people to feel safer would be labeled as racist, and “privileged” — or both. They would be be proverbially run out of town on a rail.
To then suggest the city use that data to do “something” is to exacerbate the problems. What would the city do? In some minor cases it might mean some streetlights get added or fixed. But by and large it would be more cops or patrols. In an area we already established as likely to be inhabited predominantly by some form of minority. The accusations of profiling would be flying like a flock of seagulls chasing a pelican with fish. Tensions would get high and someone would probably get shot. There might even be riots and speeches and political investigations.
And what should be a good technology being used for the right reasons would get trammeled.