https://blog.mailchimp.com/how-gmails-image-caching-affects-...
So I would say it is not 100% anymore, but, no alternative approach is out there either.
AFAIK Google does not pre-load images (for example, even if no one opened the email Google could request the image). This would destroy open tracking since you'd get a lot of false positives.
Since "who" opened is already encoded in the image URL, you still get this info.
Anyhew, there's a lot more people do with these tracking images - for example see KickDynamic.com. You can change the image at time of open (to say reflect the user's location, which you deduce from the IP). This can't be done if it's gmail, since you don't have the client's IP.
It's kind of strange that Apple doesn't block (or anonymise) images though they're so hot about privacy at the moment.