Tanins yes. Not sure if all leaves would contain cyanide???
I know however that some plants will contain much more tannins than others.
All plants from the Prunus genus are known to be at least risky to livestock (some in small and some in large quantities).
E.g laurel, cherry, plums, peaches etc.
Apple and pear leaves and safe to eat.
Also goats can tolerate quite a lot more tannins than sheep or cows but even cows will eat leaves from oak or even walnut which contain a higher level of potentially harmful substances.