In my city, and I assume many others, there's an informal landlord's group that shares lists of problem tenants to avoid renting to. While problematic, I wonder if it's made any impact.

Usually this is handled with credit reports right? It’s only when the state forbids landlords from demanding credit reports that informal networks are necessary.

In general as a tenant you can only get away with not paying rent once (until eviction happens, no one will ever rent to you again without federal or state assurances), and as a landlord you will only skip the credit report requirement once (because your first tenant is going to be a deadbeat who screw’s you).

In cities with excessive tenant protection laws, sometimes landlords will negotiate agreements with deadbeat tenants in which the tenant agrees to leave and the landlord doesn't report anything to the credit bureaus.

Credit reports do not have a section for "plays music loudly" or "secretly smokes by the bathroom window".

They can have a section of public records if anything rises to the level of filing with the courts.

And for things that don't arise to that level, but would still be massive red flags for prospective tenants?

Why's that anything to do with you. Call the cops.

"I had a problem, and I thought, 'I know, I will call the cops!' Now we have two problems."

When the people is your car got stolen, you have two problems

When the problem is lower class people are playing music too loud, the cops will solve it one way or another.