We’ve had pretty good luck with fishing line in the past too. If it’s strung out in the open, in a yard or field setting, where deer can clearly see there are no tree limbs to touch them. Something otherwise invisible touching or pressing against them, my theory at least, it spooks them.
I used some of the cheapest and thinnest I could find. Which if they happened to be running towards, they’d snap it and still end up eating what I didn’t want them to. If they were grazing and more slowly brushed against it, they’d shy away from it though. We’ve witnessed them doing so.
My orchard area is now surrounded by electric fence. But we do still put fishing line around my wife’s Daylily patch and some other flower areas, quite a few years of success with that at this point.