I remember reading Protom were working on their own notifications backend, to stop relying on Poop Play, like a year ago(?).
nostr:nprofile1qy2hwumn8ghj7un9d3shjtnddaehgu3wwp6kyqpq834th4ryhcr6z6grm07jmdr4cc5wdl0wf2kvs3dmuc979fv4fpfshrvyn3 nostr:nprofile1qy2hwumn8ghj7un9d3shjtnddaehgu3wwp6kyqpqtva8g98fx0fks0pt38vr75tv02t30qd5ev7n5v7fcpl6t44hf7kq5fm8mr nostr:nprofile1qy2hwumn8ghj7un9d3shjtnddaehgu3wwp6kyqpql024zsdxytpz3q97ngj068xkt42v7jklvl80yfm5f4vukfrf97gs52r26y nostr:nprofile1qy2hwumn8ghj7un9d3shjtnddaehgu3wwp6kyqpqwjwj5r9ytyhgg7nwmy75t8xrhphw6c8kq94fuv0qdjg6g0fruffqntd6su
All Proton apps work fine on GrapheneOS. Proton Mail requires Google Play for notifications at this time. You can install sandboxed Google Play for that.