20-30ms would be noticeable to a trained eye. Speedrunners often hit buttons frame perfect during a 1/30th or 1/60th second window. Many people today are playing on 120-240hz monitors. Additionally, if you're training on gameplay from X game and that game has theoretical ghost frames that the player doesn't see / shoot at, then the hack shouldn't shoot at it either.

It's a bit of a chicken an egg problem.

Speedrunners are anticipating the events though, and perform a remembered sequence of inputs in response to an earlier stimulus. Nobody can react to a random event within 1/60th of a second (17 milliseconds) - average human reaction time is 200-300ms. But I guess you would still visually notice a visual artifact of enemies flickering in and out of existence at 20-30ms.

Speedrunners do that via timing, not reaction. The fastest possible human reaction time with any level of consistency is somewhere around 100-120 ms, so if you reacted to 'ghost enemies' faster than that you would be definitely cheating.

On certain minecraft server anti cheats, this used to be a thing. People used to use a hack called "Kill aura" which automatically attacked anyone with a certain distance of you. Minecraft server anti cheats implemented a system which inserted an invisible fake player circling around the player, and if the play consistently hit the fake player they got banned. This system worked pretty well for a while until cheat developers realized they could pretty easily determine the real from the fake anti cheat inserted players.

There was a similar technique used to prevent botting on MMOs. The game would drop extra loot in a way that humans couldn't see or interact with, but bots would pick it up. Once picked up it couldn't be dropped and wouldn't stack in inventory, eventually making the toon worthless.

They could just modify the cheat to filter out enemies that appear for less than two frames .

True, but there would be a lag of days to weeks before the cheaters adapted. You could bust everyone in the mean time.

Valve is actually doing that. Sometimes when they update VAC, they intentionally do not ban anyone for a while to catch as many as possible. Not unlike the police is handling drug dealers.

Sadly that would be game breaking, seeing an enemy for a frame can totally break your game-sense.

You will see it after it's gone due to persistence of vision, even at <10ms (no idea what the actual number is). Mind 20FPS is longer than a frame on even a 60Hz monitor.