It’s a somewhat convoluted story. Here are some links

• https://proton.me/blog/climate-activist-arrest
• https://old.reddit.com/r/ProtonMail/comments/ydcek3/what_became_of_the_french_climate_activist/
• https://www.wired.com/story/protonmail-amends-policy-after-giving-up-activists-data/

The takeaway is when he logged into his Protonmail they logged his IP address which helped track this individual down. But note that Reddit thread I linked. I also cannot find that much information about “what happened next,” or the details of who was arrested and why.

There may be other examples, but this particular case kinda hit the rounds back when it happened.