Visitors using a browser in Privacy Mode will not be counted by Matomo if Matomo is set like this:
Matomo interface >> ⚙️ >> Privacy >> Users opt-out >> "Support do not track" >> Enable
Only visitors from desktop Firefox version 103 were completely invisible to Matomo. The other four visitors were counted by Matomo as a visit.
I tested this for visitors using five different browsers.
Matomo correctly records no visit for visitors in this browser:
Matomo incorrectly records visits for visitors arriving in privacy mode from the following browsers:
Maybe those browsers don't send the DNT header in incognito mode. But anyways: DNT's working group has meanwhile be discontinued in 2019 (see https://www.w3.org/2011/tracking-protection/), so not sure if we maybe shouldn't consider to remove it fully in the future. Safari already removed DNT support in one of their latest releases.