Tryptamine I just searched for the IzzyOnDroid repo and in fact, as you stated, there is the Proton Mail GitHub's apk right now. But when I wrote the post the other day the IzzyOnDroid repo appeared to me like it was deleted or not available, I don't know why. Sorry.
On F-Droid, I still cannot find any Proton app apart from Proton Pass and ProtonVPN, so how come you say it has been available on F-Droid for a long time?
Also, it's true that the GitHub repo does not contain Google's FCM blobs, but their compiled apk in the release page does contain them.