Certain units in NYC (about 1mm) are always rent stabilized. They’re rent stabilized because that was the agreement the developer made when getting permissions to build the apartments or more likely because they took advantage of tax abatements and benefits that required a certain percentage of rent stabilized apartments.

Mamdani’s stated policy is to set the y/y increase in rent for already stabilized apartments to 0% as opposed to the 3% average over the past 4 years. And in recent interviews he also has clarified he’s only promising it for the first year to counteract historically high rent increases over the past few years and subsequent years will be addressed like they always are, based on an analysis of the rental market.

Btw, the rent freeze was done at least 3 times in the 2010s with a 0% increase y/y, so this is not even new policy.