Income tax does affect what people choose to earn - “Why work harder for diminishing returns? I’ve got enough now...”
In truth, it more affects how they choose to earn it. Taxes affect behaviour. They are not really a disincentive to earning, they just promote those forms of earning which are less subject to taxation.
To stay employed?
Do you think an Einstein or Edison would knock off work early to avoid being bumped into a higher tax bracket?