It’s probably because Proton changed the repo. I was grabbing their release via Obtainium and noticed the version I had was way older than the version on the Play Store. Had to update my Obtainium entry for ProtonMail to the new repo: https://github.com/ProtonMail/android-mail
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