Of course it still depends on y! It is just that y was zero in the example that you just set.
Perhaps you wish to set a default value for y, so that if y is not provided, then it uses zero? Learn to use nargin to test if a variable was not provided in a function call.
help nargin
For example...
function z = f(x,y)
if nargin < 2
y = 0;
end
z = y.*(x+y);
end
