Maybe the first but not the only one. Ltt.rs (an email client using JMAP) does this as well. BTW you can also directly deliver WebPush notifications to FCM servers. No need for a proxy/rely run by the app developer.
Ltt.rs has support for both UnifiedPush and FCM and is fully open source. The code difference between UP and FCM is very very minimal since - as I said - both are just WebPush endpoints.
Ltt.rs has support for both UnifiedPush and FCM and is fully open source. The code difference between UP and FCM is very very minimal since - as I said - both are just WebPush endpoints.