[1] https://twitter.com/rjonesy/status/1401993816001978375/photo...
Compare this with Apple Mail which proxies emails from a different, presumably non-Google IP address and which does so only when an email is downloaded in the background. So while you can't track IP address, yes, and you never could set cookies that I'm aware of without clicking a link first, this means you can still track "downloads" of your email to a local client, just not "opens" - and if your Mail app already downloaded images when the email was downloaded, then it's possible it won't even change that - you might not have been tracking opens this whole time... maybe.
I searched around and found some articles that makes the same claim[1], but in my own testing that doesn't seem to be the case (ie. I had to click on the email before image would start loading).
[1] https://sendloop.com/articles/the-effect-of-gmail-image-prox...
Mozilla/5.0 (Windows NT 5.1; rv:11.0) Gecko Firefox/11.0 (via ggpht.com GoogleImageProxy)
A lot of people assumed this is what Gmail did, but it wasn’t actually the case.
(And no, I don't trust Apple not to associate this data with a user's Apple ID and datamine it in the future - if your country has lax privacy laws Apple will exploit it till the law says otherwise.)
Edit:
Here's another perspective - now, even if I don't use Apple's iCloud backup or email services, Apple has found another clever way to learn about some of the marketing emails I receive. That information is very valuable.
Given the wretched state of privacy laws in the U.S. that seems an uncharitable position. Apple has far more business motivation to treat its customers well in that regard than to try to squeeze money out of their data.
Although you’d think they’d have motivation to treat developers better than demanding a 30% cut, so there’s that.
However, I'd like to see the Privacy Policy (if it's available and in plain English) for this feature before concluding that they're not immediately discarding this data (or even just where I get my mail from, which is also very useful information to further lock me in.)