With the caveat that I’m not incredibly familiar with judicial dueling practices through history. Those got weird over time.
For those unfamiliar with the term, a judicial duel was a means of arbitrating a dispute. It’s a direct ancestor of modern legal systems. Over time there was a move away from deciding who was right via killing one of the parties or their representative. Feel free to insert a cheap lawyer joke here.
What little I know of the German Dueling Shields was that they were were intended to be used with a sword. The manuals I’ve seen do have techniques intended to be used if the sword is lost or, and otherwise unavailable. I don’t know if that was for certain kinds of disputes or if they were contingency techniques.
Shields that are specifically designed to harm the opponent are real. But, again, they’re designed to harm the opponent, but they’re not designed to be used alone.