Stinging nettle is known to have been eaten by the celts. It can be boiled or picked, which breaks down the stinging nature of the plant.

Also, cook books are a modern invention. You're not going to find a collection of thousand year old recipes by looking at written records.

While cook books are modern, we do actually find thousand year old recipes [1].

[1] https://eatshistory.com/the-5-oldest-recipes-in-the-world/