[help] What do you think about "do not pet" or "in training" patches for dogs harnesses on dogs who are not good with people or dogs coming up to them?

I have a 3.5 yr old Gsd who is very fearful. We have him on mess and have been working with a behaviouralist and he is coming along but slowly. My friend got her dog a harness with "do not pet" on it. Would this be something that would be good for my dog To help keep some space around him from people, or would it just worry people? He's intimidating enough we don't get too many people approaching us...but it is nice to let people know I don't really want people coming up to pet him. I'm just torn if this would be a good thing or not. What do you think?