Both auto-forwarding and auto-reply are paid features, which makes cancelling & switching much more difficult. Gmail is a breeze comparatively. I highly recommend against using their addresses (e.g. protonmail.com, proton.me, pm.me)
Email forwarding is available for everyone with a paid Proton Mail plan.
(source)
their point is not the custom domain usage, who cares about that, for that you need a domain to begin with and its not that common to have a personal one. but that you can’t set up automatic forwarding without continuously paying. that makes switching considerably harder for the everyday people.
Then they should change their title to say free email addresses only.