2 points
If by prefetch you mean the server grabs the images ahead of time vs the client, this does not happen, at least on amy major modern platform that I know of. They will cache once a client has opened, but unique URLs per recipient are how they track the open rates.
2 points
Apple’s Mail Privacy Protection does this. See https://www.reddit.com/r/privacy/comments/pt9ycv/apples_mail_privacy_protection/ for a post from three years ago talking about it.
I don’t know if any other major providers take this approach but Apple / iCloud is definitely one of them.
