I thought "holstered" shields do protect your back since beginning of M&B. I believe there are also animations and hitdetection for arrow hits on "backworn" shields, and sounds for hitting wood, when backstabbed or sideswinged on a backworn shield. Not 100% shure. Just would be nice to know if I was wrong all the years.
If its not already the case: Holstred Shields on the back really should protect your back (not your head - your back!)
The animations and shit are there for it, but they are not in crpg
I played mods (and i think native might also have it) where shields on your back block arrows, and provide extra armor for hits in your back by melee (though not blocking them).
I suppose those are disabled here for balance reasons, though i don't know if thats still legit with the amount of ranged on servers like eu1.