Though you are warned not to eat rhubarb leaves, it is safe to put them into your compost. Rhubarb leaves are toxic, but once they're in the compost, they decompose in a way that won't harm plants or humans.
Rhubarb leaves can be poisonous if eaten or cooked into food because they contain an organic compound called "oxalic acid." However, once placed into your compost, the rhubarb leaves begin to decompose and the levels of this compound are low enough that it no longer poses a threat to humans. Also, oxalic acid is not easily absorbed by plants, so compost containing rhubarb leaves can be safely used on vegetables.