They are literally one of the best places to find SH**,,, pun fully intended.
I have dug everything from chamber pots to coins to tons of medicine bottles,,old blob top bottles etc. from outhouses.
And unless someone just "used" it,,, the "moondirt" is safe as regular dirt,,, within reason of course.
Look for depressions in your yard,,, if old enough,,, usually every coupla years a new one was dug.
My place was built in 1850 and I have found only 2 so far,,, there has to be at least 4 or 5 more there.
Go for it,,,, just be warned,,, they can go pretty deep,,, up to 20 or so feet in some cases.