Sterile refers to whether it has germs. I'm talking about other kinds of bad, like caustic or causing chemical burns.
As an example, bleach is sterile, but straight bleach is bad for your skin.
As regards ashes, lye can be made by dripping water through wood ashes.
Lye may be sterile, but getting it on your skin is definitely a bad idea.
That is why I would be concerned.
OK, so ashes can sometimes be used safely.
As long as the chickens have other options as well, so they can avoid it if the ashes cause them problems, that makes sense to try.
I am much more aware of ashes --> lye than of other uses for ashes, so that's what occured to me first.