Where I live (central Oregon), this is not the case. More than 80% of our homeless population are people that were born and raised here, and were priced out of housing and onto the streets due to housing prices skyrocketing to eyewatering levels. It has tripled in the last ten years, with a 1-br apartment going for under $600 in 2013 now going for $1700. The homelessness here is homegrown, not imported.

Portland is better housing-wise, but not by much. Considering how most people who are both homeless and on drugs were homeless first, then turned to drugs, I think this is a strong confounding factor. It's hard to saw what effect decriminalization has had on drug use when it's adjacent to a housing crisis that is manufacturing more homelessness and drug use all on its own.