Plunger stuck inside tub drain
I would stay away from the acid because it can also eat your metal pipes. If the drain hasn't been used for years it is possible that it's rusted in place. I would try CLR  as mentioned above. The balloon trick is clever. Slow process but it MAY work. Most older drains are brass and the stopper is also brass. Corrosion is more likely the culprit. Once you get it free you can pull the entire assembly out through the overflow hole to clean or replace it. Good luck!