The shear bolt is an overforce mechanism plain and simple. Its one and only job is to protect downstream components from damage. Typically it is located just ahead of the gearbox as others have stated.
If it was positioned at the tractor end, all components downstream will be protected, the u-joints, the shaft, the gearbox, etc..
I take a more risky (to some) approach to driveline safety. At the tractor end, when the implement is in use, are you likely to come in contact with an unshielded u-joint? You have the top link, both 3PH lift arms, 2 draft arms, all the structural steel of the implement hitch, etc. all preventing you from contacting the rotating shaft easily.
On the implement end, there fewer obstacles between you and harms way. Are you or any other person going to be standing with their toes under the flail mower and within reach of the driveline while the PTO is engaged...let's hope not.
Using this logic, if it was my tractor and my dilemma, I would remove the guard on the tractor end and leave the shear bolt where it is.
If you were pulling a hay baler from the drawbar, the driveline would be more exposed and I'd be much more concerned about accidental contact with exposed components.
Everyone has an opinion and now you have mine.