Based on my search this year, that isn't necessarily true. Here in Denver, all of the insurers that were offering PPO plans in years past have left the individual market (not just the ACA exchange), save one: Anthem BCBS.

Availability appears to vary widely, both from state to state and within states. If I lived in western Colorado instead of on the Front Range, I'd have a couple of PPO insurer options. If I still lived in Minnesota (where my previous health insurance policy was written), I'd have dozens of options from at least four different insurance companies.

I really miss my insurance policy from BCBS of MN.