How to disable sidetone (voice feedback) on USB headset on macOS Catalina?
I got an answer from the Plantronics (Poly) support team, all their products have sidetone enabled.

On macOS, the driver (Plantronics Hub) they provide is not that good and doesn't allow to disable sidetone which is probably firmware enabled.

I've also tested the headset with the 3.5mm jack on my smartphone and the sidetone is not present that why I tend to think that the sidetone is handled by either Plantronics Hub or headset firmware.