According to quick wikisearch, all cities except NYC, Boston, Cambridge, Chicago, Philly and SF
> Several major cities in the US use these in their taxi licensing systems, including New York City, Boston, Cambridge, Chicago, Philadelphia, and San Francisco.
I think you might have misinterpreted the sentence here: these are cited as examples, not at the only cities in the US to have such a system in place.
"Denver taxi medallions" seems to return nothing indicating they have one.
Los Angeles doesn't seem to have one either