Is this the brake pad wear sensor clip? If it is then it would be safety relevant if the warning does not come on despite low brake pads. If the pads have plenty of material left, it would still be safe to drive though.

Manufactureres dont tend to fit components that dont have a valid use, over the production run it could cost them a huge amount of money. If that part cost is say just 1euro and they make say 750,000 cars......go figure.

My own view is that if some design engineer somewhere in Germany thought it was required who am I to argue. Maybe your car has not yet seen a condition where the part comes into play, it doesnt mean it wont sometime in the future.

Perhaps the most obvious question is...... if the garage say the car is still safe, why are they failing it?

MOT appeals tend to be over condition of parts. I doubt they will fall on your side over something thats missing allthogether, especially part of the braking system.

I would just suck it up and move on, but if you think its unfair....there is no harm in trying an appeal.