I sometimes get messages such as the following from newspapers' web sites (Washington Post and New York Times):
We noticed you’re browsing in private mode. Private browsing is permitted exclusively for our subscribers. Turn off private browsing to keep reading this story, or subscribe to use this feature, plus get unlimited digital access.
Actually, I am a subscriber to both of these papers, but I'm not paying a subscription for the privilege of being tracked. Ironically, non-subscribers can hit the page and read (in non-private mode), so in a sense they get treated better than subscribers. (The can read anonymously, although they get tracked.)
How do these web sites know that I'm in private mode? It doesn't seem to have anything to do with cookies. (Disallowing cookies but visiting the site in non-private mode doesn't cause the problem.)