Phead
I agree that it's annoying. Maybe the easiest way to disable the notification would be to disable it completely. Just go to the app's notification settings and turn off "Incognito".
Other browsers do the same thing, like Edge and Brave, so it would most likely be an upstream Chromium thing. I can't say if it's intended, but I personally think it would be kind of dumb if Google actually meant to have that notification continue being around even after the browser has been closed.