TL;DR They don’t log IP addresses. But they can be compelled to by Swiss law and they cannot NOT oblige as it’s trivial for them to do at various levels in thei

TL;DR They don’t log IP addresses. But they can be compelled to by Swiss law and they cannot NOT oblige as it’s trivial for them to do at various levels in their stack without even needing to modify their software. So they advise you to use their onion address if you need to anonymity. Don’t know why they can’t plonk a tcpip->tor->ProtonMail reverse proxy in front of their infra offering this facility to every connecting client, and transparently. After all, their services (including ProtonVPN) already support tor to some extent.