I leaved a comment with their "Report a Bug" functionality. And they have seem to give this repo more care since then.

A supposedly open source app, with no public updated code should be considered a high priority bug.
https://github.com/ProtonMail/ios-mail/
That repo was updated almost 6 months ago, however, multiple versions of ProtonMail have been released to this day.

Their reply:

Thank you for contacting us.
We have forwarded this information to our appropriate team so they could look into it.
Many thanks for bringing this to our attention, if we have any relevant information we will follow-up on this support ticket.

…and the App Store version is currently at version 4.0.5 already while this repo’s latest commit fa94f3e is for version 4.0.0.

The team listed in the README (@zhj4478, @abjurato, @henrybear327, @emnproton) are not listed in the ProtonMail GitHub organization and only @zhj4478 actually appears as a contributor.

Clearly, Proton is ad-hoc mirroring to this repo rather than directly developing in public or automating releases to it. That’s their prerogative but it’s not as transparent as a process as it could be.