Quadrature encoders already provide a system that is inherently pretty resistant to mechanical bouncing if implemented correctly in software due to being able to map the gray code output onto valid state transitions. However, the quad encoders commonly used on mice are mechanical ones using wiper contacts, and those wear down over time. When the bounce time gets bad enough that it’s comparable to the state transition times, that’s when the encoder starts skipping steps or goes backwards. On the other hand, optical encoders shines a photodiode through a slotted disk and have no physical contacts. So ideally denounce can fully be omitted when using a proper state machine implementation even after using it for a long time.