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%20Policy
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
I was/am in the same boat, but I quickly gave up for the following reason: Why bust my ass trying to give my business? If Lowes blocks me then f*ck em I'll take my business to Home Depot. It's their loss.
I can't give you an answer, but what I can tell is that, unfortunately, being on iOS, you won't have a lot of options when it comes to FOSS apps.
Apple charges developers $100/year to be able to publish on the app store, so that gives very little incentives for iOS developers to publish their app without any kind of monetization, whether via a subscription, ads, or mass data gathering.
I have a lot of respect for FOSS apps on iOS, because not only do devs offer an app FOSS, but they're paying that $100/year out of their own pocket.
For the known top trackers, it's to have more control over nefarious users such as duped accounts, banned users and potential law enforcement trying to sign up. Sure it's not a bulletproof solution to deal with bad actors but it's still an effective way. You gotta remember that these trackers are free (yet private). The admins/operators don't get paid, so last thing they want to do is waste time dealing with such users.