There is clear social harm to have cities filled with empty homes while there are people who want homes and can't afford to buy any on the market or are forced to commute long distances.
Using real estate to store money and other speculation does harm to people who want to participate in the market to actually use the limited fundamentally necessary resource.
It makes good sense for property owners who actively use their property themselves to have more rights than those who seek to profit from market conditions or others' use.
Having paid taxes is a weak justification for any freedom.
If you are a landlord, you need to rent the place out at the current market price. If you do not want to do that, you can sell the property and get out of the landlord business. Just squatting on it and leaving it empty/boarded up distorts the local property market and should be heavily taxed to discourage absentee landlords from interfering with local markets.
Just to put it in a US perspective, Barcelona as a whole has 10% higher population density than Brooklyn (41,000/sq mi vs 37,137/sq mi).
At night, you walk through that place, its quiet like abandon for very long time, and there won't be any retail stores / business nearby since only ghosts and wondering dogs/cats will be there.
But I also wondering, if the city mayor decided to make this bill work, he need the local city council pass the bill, will this make him lose in next year's election?