I have never seen one used.
In Judo (my ex art)...we sometimes had new people come who wore shin guards, or perhaps they wore them after a session or two
...it didn't matter, no-one was 'hitting' shins to hurt them and frankly they just got in the way (for the wearer) and they soon stopped using them when they saw it was making no difference at all, except focusing their mind in the wrong place.
What I never saw in more than a decade of judo, was a groin guard. Of course there were occasional accidental groin injuries, but the groin is not a 'legal target' so injuries (or just plain shock) were so rare that it is never worth catering for.
To me the tsuba is like the groin guard, no-one is going to try and chop your fingers but we all accept it may happen on a rare occasion.
However if I thought someone was going to target my fingers I would be wearing Rachel's hockey gloves. And the attacker better be wearing a groin guard.