Here they literally say, “Proton owns the proton.ch domain.”
@protonprivacy uses it for their staff. That is why it is not available for e-mail customers.
Use proton.ch domain
Some users want to use a domain that is under Swiss jurisdiction. This is why Proton lets us end our addresses with protonmail.ch. However, that domain is quite long. This is why Proton added pm.me as an alias and then added proton.me as an optional ending. The .me domain is under the jurisdiction of Montenegro, who is a member of NATO and is joining the European Union. Proton owns the proton.ch domain. Please let us end our email addresses with proton.ch instead of proton.me.
The Voice of the Proton Community (protonmail.uservoice.com)