Doesn't. You can do almost nothing to hurt a cast-iron pan itself.

It can destroy your seasoning, though, and require you to "fix" it by thoroughly cleaning and re-seasoning it. It's not that difficult, but generally you want to retain your previous seasoning as it's much easier and it gets better with time.

People also think soap and water "damages" cast iron, and that's not true either.

http://www.seriouseats.com/2014/11/the-truth-about-cast-iron...

Anyway, prolonged rusting or breakage are the only ways to "destroy" cast iron.

Deglazing is pretty hard on the seasoning of the pan (basically polymerized fats from cooking), which means your stuff will probably stick next time you cook with it.

I actually use exactly the method above for caramelizing onions, and I use either an enamelled cast iron pan or a tri-ply stainless/aluminum pan. Both work well, but I think the results seem a little better with the cast iron. That may just be because I can go straight into making onion soup in the same pot, so every bit of the sticky onion stuff makes it into the soup.