Serious question that I don't know the answer to: when and where, in the US, has rent control not been accompanied by constrained supply?

I'm not using your phrasing, "artificially constrained supply", because it seems like artificial could have a very squishy definition. A lot of economists these days are saying that rent control leads to lower supply, but they seem to mean it as more of a natural consequence (i.e. without necessarily needing the kind of NIMBYism and red tape that gets blamed for low supply in the largest cities with rent control).