lime
Normally shale does not harden, but there are also calcareous shales.
If slate contains lime, then a lot, so definitely test it!
You can check relatively easily whether yours is hardening – pour vinegar or citric acid on it.
If it foams, lime is in it and it hardens (see lime content). There is lime-free slate in the Taunus, among other places.
oil
Shale can also contain oil, and this is far worse than lime. Brown and black discolorations are an indication of oil.
You should definitely clean the stone first and then leave it in a water tub overnight.
If the water has an oil film the next day, throw away the stones!
pyrite
Slate with shiny golden metallic inclusions (pyrite, FeS²) should also not be used.
The pyrite itself is not the problem, it is quite harmless, but the inclusion may also contain traces of cadmium, arsenic etc., so stay away!
Conclusion:
Normal black slate (without lime, without oil, without pyrite, mostly smooth and clearly layered) should be harmless.
Most shale is fish safe, but of course you can pretty much fall in without a test!