What is a ghost ban or hidden reply?

A reply can be pushed behind Show more replies or hidden from non-followers. It looks normal to you, which is why this is the most commonly missed restriction. X Shadow Ban inspects a sample of recent public conversations and compares the replies visible in the thread with the account's replies.

A single buried reply is ordinary ranking, not a penalty. The check needs enough public conversations to measure; with too few, the honest result is untestable rather than a pass.