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.