It is now 2024 and the Proton Calendar mobile apps aren’t opensource and haven’t been since 2021. While web source code is available in their webclient repo this still violates their open source guarantee on their landing page and dedicated open source page.
All our apps are open source and independently audited by security experts so that anyone can use them, inspect them, and trust them.
For over two years now they have been saying they will publish the repo on Github as referenced above by IksNorTen and again 1 year ago and again 3 months ago.
To me this seems very strange if the Proton Calendar mobile app codebases are mature and well-written they should already be well documented. However the fact that they have been unable to do something as simple as publishing the repo after all of this time and that they still represent all of their apps as open source instills distrust.
In summary, I too believe the Proton Calendar recommendation should be revoked, reconsidered or at least caveated in some manner.