The following FunctionRange

(*a=((1-x) y)/(1-y z)*)
FunctionRange[{{((1 - x) y)/(1 - y z)}, 0 <= x <= 1, y > 1, 
   0 < z < 1}, {x}, a, Reals] // Simplify

returns

y > 1 && z < 1 && z > 0 && y z != 1 && a y z <= a && a <= y + a y z

I'm struggling to see how to illustrate the values of x, y, z and the resulting values for a that are excluded.

Appreciate any hints or tips.