UnifiedPush open-standard push messaging complements degoogled android-compatible phone OS's such as LineageOS.
People who do not want to depend on Google or have them control our devices are using android-compatible but not google-controlled phones, a.k.a. “degoogled phones”. We have been asking (ourselves) for several years if we can have google-free push notifications. Thanks to the developers of the UnifiedPush standard, the answer is now, “yes!”
The open standard UnifiedPush.org has now been created. While not a large number yet, a useful handful of apps already support UnifiedPush, including several matrix and fediverse apps. For its servers and the associated client-side “distributor” component, there are multiple successful implementations deployed.