Let me qualify as saying it would create more problems for the hay cutter than the occasional and most likely infrequent fawn.
For such a bar to have any effectiveness, it would tend to lay the hay down making it harder for the swather to get a good cut as well as it being harder to steer clear while turning tight corners. It would also be a hindrance operating on uneven ground and going over terraces where it would be vulnerable to being run in the ground subjecting the whole machine to potential damage.
Ok, I see your point. I know around here onlya few farmers use flush bars and time their cuts right before fawns are born. Most however do not. Again I'm talking about your small time farmers cutting hay for their cattle, usually less than 40 or 50 acres of hay total. I think in many (not all) cases this type of farmer could help prevent some damamge if they really wanted to.
It happens, unfortunately.