Spacer lifts are just solid aluminum so they will not settle. But all springs will sag or settle a bit over time. So to say the lift will settle is incorrect, but the springs will eventually lose some of their height whether or not a spacer is used.
If you got 2" of lift from a spacer and a year or two later you measure it again and only get 1.75" then the cause is spring sag, not the spacer lift.