Is it safe to deglace a cast-iron pan?
A metal pan will not crack simply from cold liquid.  If you heat it up to a very high temperature and submerge it in cold water, that's not such a good idea.  But deglazing is just a tiny amount of liquid.

Now, aluminum is another story - if it's hot enough and you pour cold water (or cold anything) on it, it can warp, even with a relatively small amount of liquid.  And for Teflon and other "coated" cookware, you can ruin the coating that way.  But heavier steel or cast iron - no way.

If you're really concerned, just keep the deglazing liquid at room temperature.  Deglazing is usually done with vinegar and you keep that at room temperature anyway, right?  I deglaze my CI all the time with cider vinegar and have never experienced even the tiniest crack in 10 years.  I'm pretty sure it's safe.  It's great, actually; CI deglazes very easily, you just need a small splash of vinegar.