Do things described in the article, only track forwarded emails, or if the text is cut and pasted?

Would blocking html work?

I already tell clients to only use ProtonMail to communicate anything sensitive /burn it into CDs/DVDs.

https://www.militarytimes.com/2019/05/17/secret-tracking-device-found-in-navy-email-to-navy-times-amid-leak-investigation-raises-legal-ethical-questions/

I think to block loading remote images in emails would probably help. Also VPN.

I think this technology is a pretty common marketing tool. I'm sure that all of your big data usual suspects log where the remote images are loaded from.

I'm really not surprised by it, but the fact that a prosecutor is using it during an ongoing case doesn't pass the smell test. Appearance of impropriety kind of deal.