You're getting two different things confused here.
Student horns from the 1960s through 1980s or so, plus some pro models, have had nickel PLATED keys. The keywork underneath is the plain old brass. These I believe were offered so the keywork would stay shiny longer under heavy use (stripping polishing and respraying just the body is a lot cheaper for the school district than doing the keys too). There's no functional advantage or disadvantage beyond that.
Martin saxophones for a period in the 1930s had the keywork made of SOLID nickel alloy. (Most unfortunately this is often referred to as "German silver" or "nickel silver", but it has zero actual silver, element Ag, in it.) This makes the keys more resistant to bending than brass keys of the same dimension (higher yield strength), more resistant to wear so pivot points etc. get loose more slowly (higher hardness) and might even give it a small amount additional stiffness (higher modulus of elasticity). I believe that Martin actually lacquered over the nickel alloy keywork to prevent it from the discoloration and greyish appearance that "nickel silver" will take on with time.
Some people will refer to "nickel silver plating" which basically doesn't exist. If it's nickel plated it's elemental nickel.
Other manufacturers at times have used nickel alloy for some components, especially either touch pieces (Buescher used it for some thumb hooks) or long long rods (Buescher, King, and Selmer have all done this at some time). I'm not aware of any manufacturer other than Martin using solid nickel alloy for the complete keywork of saxophones. That's a little strange, because almost all silver plated flutes use nickel alloy for keywork and tubes rather than brass, and at least a very large fraction of clarinets use nickel alloy for keywork whether they plate the keywork with nickel or silver (I have an older clarinet with unplated nickel alloy keywork as well).
I know Keilwerth made a few saxophones with nickel alloy bodies in the 90s or so.
The "German silver" nickel alloys and elemental nickel plating are visually distinguishable from each other pretty easily (and from silver) when polished, but when tarnished it might not be so easy.