I often think the best way to defeat email open tracking would be for a mainstream email client to prefetch every image when a non-spam email is received and cache it for 72 hours or so.

Every email gets flagged as “opened,” so the flag is meaningless, and recipients can see the images without triggering a tracker.

I worked for a short time for an American company. They had periodic phishing test from Mitnick. The links in those emails was not to be clicked as it would trigger a mandatory training. The emails also had a header saying they were a phishing test, so I deleted all those emails in a filter.

The company also ran a mail filter called Baracuda or something similar that followed links in emails to see if they were malicious.

I was quite annoyed when I was called to do the mandatory training as "I" had clicked a link (on an email I hadn't seen) and more so when told I had no other recourse than to sit through it.

I resigned shortly afterwards.

Some of the big providers already do this, notably Apple and Gmail:

https://www.litmus.com/blog/gmail-prefetching-images

That still provides “human” vs “bot” feedback to the sender.

An automated system processing emails isn’t going to be fetching images or rendering attached SVGs.

I think this is what icloud does. Seems like an easy way to make tracking useless if every client did it.

That is still signal that the email address is valid. I'd prefer something like the server immediately sending a SMTP 550 5.1.1 (unknown recipient error), for anything that's immediately recognized as spam (or marked as spam in the past by the user). That gives no signal at all and might even persuade some scammers to remove your email address from their list.