Some of the big providers already do this, notably Apple and Gmail:
https://www.litmus.com/blog/gmail-prefetching-images
Gmails prefetch is terrible for privacy because it honors http cache headers, which means tracking companies simply use a "no-cache, must-revalidate" header to defeat it.
alt Hacker News
Gmails prefetch is terrible for privacy because it honors http cache headers, which means tracking companies simply use a "no-cache, must-revalidate" header to defeat it.