Probably not fatal, we see all sorts of crap in ruminant stomach. But there's so much you'd think that there might be an effect on growth because they can't get enough grass in. Be interesting to see how it compared to available rumen space. It would only be an issue if it could get through into the abomasum and small intestine, but the way the rumen circulates fluid/solids, it probably wouldn't get there.
I find it all the time in fallow.. Its pretty common and normally with no ill effect. I have shot deer that have been looking very sorry for themselves and being well under the expected condition for time of year. On inspection, they have had over 2kg of silage wrap and bailer twine in the rumen. One of which was actually shot by a vet (deer stalking with me, not humane dispatch) and his take on it was the same as Buchan, its more to do with space to process normal food. A bit like a human having their stomach stapled i guess.
I have never noticed any laid about, our farmers are fairly tidy people.
The deer wander far and wide, I suspect but cant prove it, that they pull it off themselves
A motorbike event claim the deer chew plastic signs they put out
Was interested if any vets had an opinion on it being responsible for an early death