Sugar – Does sugar go bad

White sugar was commonly used as a preservative in the past, in much the same way as salt. It's wildly hygroscopic (like salt), and an excellent desiccant, so if you packed something in it, it would accelerate the drying process. This is actually the origin of fruit jam and fruit preserves, which are still common today, even after better methods of preservation are available.

Salt was more common simply because many things like eating/living in WET sugar, so you had to be careful with how much water was in the thing you were trying to preserve. But as long as the sugar was dry it's an extremely hostile environment for bacteria and little crawly things.

Kept dry, it will last forever.

Sugar does not cause sour food to be any less acidic. The difference is purely one of perception; we are wired by evolution to prefer sweet tastes and tend to perceive less of other tastes when a high sugar concentration is present.

Sugar does not ionize - it is not basic or acidic. Acidity itself is a chemical property, and only a base (such as baking soda or trisodium citrate) can actually neutralize it. Two other compounds - miraculin and curculin - actually alter our mouth chemistry and really do cause sour foods to taste sweet, without actually sweetening the food itself. But plain sugar does nothing at all - there is no chemical reaction happening. It's just masking other tastes, not neutralizing them.

The reason sugar doesn't mask bitterness as effectively as sourness is that sweet and sour have roughly similar taste thresholds, while most humans are extremely sensitive to bitter tastes. We can detect quinine (the reference solute for bitterness) in solution at 0.5 ppm, whereas sucrose isn't normally detectable at levels below 5000 ppm. The amount of sugar needed to mask a significant bitter taste is simply not practical.

If you want to neutralize a bitter taste, use salt instead.