If you are worried about something poisoning you don't compost it.
Alternatively, you could neutralize the toxicity using the same methods as when eating cassava. The simplest way (used in West Africa) is to peel the roots, then soak them in water for three days.
It might help to start by explaining what "cyanide" actually is. It just a chemical bond between one carbon and one nitrogen atom. Nothing particularly scary about that.
"Cyanides" are any chemicals that contain that carbon-nitrogen bond. Some are extremely toxic (e.g. hydrogen cyanide gas) and others are almost completely inert (e.g. the "nitrile" in nitrile rubber gloves).
Cassava is a staple food and the main source of carbohydrates in some parts of the tropics, but it is certainly toxic to humans because of the cyanides it contains unless it is prepared properly. Of course the indigenous people who have been eating it for millennia are well aware of this fact and what to do to make it safe to eat.
On the other hand, the stable "nitrile" compounds are found in hundreds of plant species, including brassicas (e.g. cabbage, which nobody thinks is "poisonous") and they have no effect on humans - they simply pass through the body unchanged.
The effects of cyanides in plant biology are diverse and complicated. For example some plants produce them as a deterrent against being eaten by animals, but in other species they can either promote seed germination, or inhibit root growth. Take your pick...
For example, here is a study showing that earthworms can neutralize the toxicity of cassava compost resulting in increased plant yield: https://www.sciencedirect.com/science/article/abs/pii/0921344996011020