It seems Proton has finally published the source code to their Android calendar app, although I’m not sure it’s actually up-to-date given that the last update was a couple months ago. It’s also still not available in F-Droid or through GitHub releases AFAICT.
The branch name indicates that this is the source code for the latest stable version (v2.29.0-332). I confirmed this is the version the Play Store currently offers.
They’re probably also doing this in preparation for the reworked Calendar app, after they finished solving the issues with the new Mail app. This also means all their core apps are finally open source.
Thanks for the updates, @byesun and @anonfox! I updated the Proton Calendar recommendation on the website to reflect this, and it should go live in the next update of the site.