The Firebase team has announced that the services supporting v3 registrations are being turned down in April 2019. (See Issue 831531 for full context.)

The Push API implementation does not create GCM subscriptions anymore, but still supports unsubscribing existing subscriptions. See PushMessagingServiceImpl::DidClearPushSubscriptionId().

UMA shows that this still very frequently happens:

https://uma.googleplex.com/p/chrome/timeline_v2/?sid=62271a60d4f1c1c5b36766c337f75528

The call needs to go, but we need to figure out what to do with all existing v3 subscriptions that are still out there first... (Subscriptions created prior to late 2016.)

https://chromium.googlesource.com/chromium/src/+/a504573