The amount of vacant units in cities where people actually want to live tends to be highly exaggerated (Manhattan is generally sitting somewhere around a 5% vacancy rate), but twisting income tax into some weird kind of tax on unrealized value is administratively messy and completely unnecessary when we already have much simpler solutions in the forms of land value taxes or even basic property taxes. Not to mention, increasing taxes on rental units just increases everyone's rent, which is a rather odd strategy if the aim is to make housing more affordable.
People really will propose literally anything except the wild concept of building more housing.