Neither the mentally ill nor drug addicts are protected classes. The best protection they will get is under the ADA, and I’ve never seen it be used to argue for access to housing for non-physical handicaps.

That is not correct. I'm sure mental illness would qualify for ADA protection with the correct doctor's note and filings. Drug addiction is definitely protected under the ADA.

(Pdf) https://www.hhs.gov/sites/default/files/drug-addiction-aand-...

Not for housing. Protection yes, but landlords are not required to not evict a paranoid schizophrenic that has threatened violence on other residents (and the eviction sticks when they try to rent later). The ADA also doesn’t prevent landlords from ignoring drug convictions from someone a substance abuser. They can’t ask you for your status, but they won’t ignore any other signals that are a consequence of that status.

Maybe… but most localities will allow you to do a background check.

As a former landlord, I can almost guarantee someone with the problems you mention will and felonies, poor rental references, and/or evictions. You can easily use those to legally disqualify a prospective tenant.