Stinging nettle (Urtica dioica) should be safe to feed to rabbits. Rabbits eat a lot of things we humans would think are impossible to eat because of thorns ect. for example blackberry/brumble or raspberry leaves (rubus varieties). My bunnies love those and eat them with thorns, no problem.
Most rabbits prefer dried stinging nettle, though. It makes a good addition to hay in winter. You could try to feed fresh young leaves and see if they eat them, sometimes they wait for a few hours before they eat it. If they don't like it you can try to feed it as hay (pick and hang to dry just like kitchen herbs). I only know the common European versions of stinging nettle, though, so should you be located somewhere else I'm not sure about your local varieties.
Stinging nettle as I know it has a lot of of protein. So it shouldn't be fed in high amounts, only as an addition in a mix of lots of different fresh greens/grass, hay, ect.
Where I live, a more natural diet for pet rabbits is recommended (vs mainly hay/pellets based diet) so I feed many different wild herbs and leaves daily. They can choose what they prefer to eat that way and leave out things that are not tasty/not good for them in high amounts.
