It's not E2E encrypted unless you're using PGP or emailing another Proton user, which is basically nobody for most people. They do encrypt your email when it arrives in a way that is supposed to make them inaccessible to them (which is more than what most email providers do), so you'd need to trust that they're not intercepting your emails and storing them somewhere unencrypted. Stuff like SimpleX/Cwtch/Signal is E2E encrypted though by default so their security is a lot better
I did some digging to see what was that I had read, and I think it was a case a few years back of Proton complying with a Swiss subpoena requested by US, investigating death threats to Fauci. Proton disclosed (limited) information about the account that sent those emails.
I think because Proton promised complete privacy, and did provide user information, it ended in my brain as “Proton is not as private as they tout it to be”.
I am not advocating for protections of highly illegal acts, but since:
PGP encryption is only good if both ends use keys, and
Proton complies with Swiss law requests
I stored Proton in my “Marketing bullshit” cabinet and never opened an account. Other than not selling my info to advertisers, it seems the same as Apple email, for example.
I think I remember it was when Proton complied with Swiss subpoena and provided user account info requested by US. I clarified my nebulous comment in another thread here.