For as long as the application is in beta-state I don’t think it will happen as that would require more attention than usual for packages in the official repository.
I am fairly certain that when proton mail application get out of beta stage - an Arch Linux maintainer will provide it just like Proton VPN.
Besides that - it is available from AUR using
pamac build proton-mail-bin
This will download the ProtonMail-desktop-beta.deb and build the package.
As you have noted - it is based on Electron - a specific version of Electron and therefore you may - rarely - get a situation where Proton urge you to update - but the required version of Electron is not available on stable branch.
I had that issue a couple of weeks back on unstable branch.
By the way, I’m the one who initially added proton-mail and proton-mail-bin to the AUR. I eventually disowned both last year as maintaining them became more trouble than it was worth.
For a long time, Proton would not push the source code to the git repo when they released a new version for months on end. That also made it difficult to track when new releases were published. Then they moved it to a monolithic repo with the web client and updated it a little more often, but still not in a timely manner.
Unless that changes, I don’t envy anyone attempting to maintain the package.