You probably need to make a request to Izzy. From what I remember he has requirements, such as max app size, which makes me wonder if maybe the ProtonMail app just became too big for him to build and host.
Here are the requirements: https://gitlab.com/IzzyOnDroid/repo/-/wikis/Inclusion Policy
Ubiquiti is business oriented, not consumer. It’d be very foolish for them to snoop on the traffic of their business customers.
Ubiquiti is also a traded company. Their stock would crater in lieu of such news.
As a consumer, besides reliability, privacy is another main reason to paying extra for enterprise gear.