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.

But the path changes with every new data element. It’s never the same, so every “prefetch” is a whole new image in the system’s eyes.

Personally speaking, I’ve never been a fan of this method because to the hosting web server it was still fetched. That might confirm that an email address exists or (mistakenly) confirm that the user did in fact follow the link (or load the resource).

I have ad and tracking blocked like crazy (using DNS) so I can’t follow most links in emails anyway. External assets aren’t loaded either, but this method basically circumvents that (which I hate).