I've never heard of deposits for reservations for a restaurant. And I've at least once made a reservation for restaurant with a Micheline star.
I'm not sure if this is common practice at least in Europe. I'm sure it's unheard-of in Turkey (and I've been lucky enough to make reservations for some high end and/or very popular restaurants).
May I ask in which countries you've experienced this in? I'm genuinely curious.