It has been shown time and time again that price floors or ceilings very rarely help a market be efficient. Quite the opposite usually.
But the classification doesn't matter to me personally at all. If the apartment went from $2400 to $2600, while it was decontrolled, the affordability hasn't changed that much. Meanwhile a unit that was $1900 several years ago could be $2400 now because they reset the rent every time the tenant switches. It is much less affordable, but it is still rent controlled. This data is not going to help you figure out if things have gotten significantly less affordable.
Which really means this is just a map of where median rents are close to the line between rent controlled and not rent controlled. Notice that the areas that are much more expensive don't show change, since everything was over the line to begin with. The places that are really cheap also don't show a huge amount of change because even after increases the units are still rent controlled. So overall it's good data but it's not telling you that much.
i.e. it doesn't matter that I'll pay market rent for a rent-controlled unit I move into today. What matters is that the yearly increase in my rent will probably not outstrip my income over the long term.
They are two different things.
"Rent Control" is something that specifically applies to residents occupying a unit prior to a specific day in 1971 and their qualified heirs.
Personally I think if we really want to subsidize housing for some New Yorkers of modest means we should do so with vouchers.
Also. http://nymag.com/news/features/housing-projects-2012-9/
The problems are a result of the political will shifting and the funding disappearing.
However, the free market has generally been a terrible provider of low-income housing solutions. There's a long, long history to this from shanty towns to tenements. That's why basically all developed countries have some type of social housing component from demand-side subsidies to public housing or regulated stock.
When we lose permanently affordable units, it is invariably extraordinarily expensive to replace them given land costs and the size of subsidies that are needed. Contemporary homelessness emerged in the late 1970s and early 1980s in both San Francisco and New York as both cities started to lose single-room occupancy hotels or SROs.
The number of units under price ceiling have an insignificant impact on the market anyway.