It can be done without causing harm, but if not done correctly, it will cause harm. When coasting to a stop or lower speeds, I can very easily shift to neutral in my car at 11 mph from second, 16 from third, 24 from fourth, and 32 from fifth. Those speeds work in my car. I do that with some frequency.
Disengaging a gear from a higher speed can be done, but I see no need for it. I’d rather wear out and replace some clutch parts than risk damaging internal transmission parts.
The gear (actually the synchronizer) needs to be disengaged while there’s no load. The easiest way to remove the load is by pressing the clutch pedal. You can also wait until the car has slowed to a speed at which the engine is at idle speed. The other way is to shift to neutral at just the right time while releasing the accelerator. It has to be done when the engine isn’t trying to make the car go faster, and when the motion of the car isn’t trying to make the engine go faster. It takes practice, but I see no reason to do it.