Yes. They are saying that if people really wanted to tell you when they open your email, they could enable it. However, they don’t. Throwing tracking pixels in are really a means to circumvent the wish of the recipient.

> The default setting for such feature is to ask the user. It makes the whole interaction more creepy.

I never considered that a consensual request prompt could be considered 'more creepy' than non-consensual spying. What about that interaction upsets you? Simply knowing that someone wanted to do it?

My general heuristic is that if people knowing that you're doing something to them would be a problem, then you should at least consider that it might be inherently creepy to do it.

The reason people are creeped out when you ask them whether or not you can monitor them is because they don't want to be monitored. Skipping that question and tracking them without their permission doesn't make it less creepy, it makes it more creepy.

I remember both Roundcube and Squirrelmail supporting them, and they are quite common.

No, there actually exists an email header to request a notification email to be sent back whenever the original email is read. This header exists since at least the 90s, and for instance Thunderbird has options to both set that header, and to honor that header (sending the notification emails). I vaguely recall that the default, at least when I last configured Thunderbird, was to not add that header, and to ask before sending a notification email.

For Outlook and Thunderbird yes, I am not sure about the support in Apple Mail and webmail clients.