Thanks for the link. I noticed some of these OEMs had started implementing their own "more reliable" PushKits, but I am guessing that the privacy implications are quite bad so it's basically a non-starter.
Privacy or not, it's the only way to send notifications to new Huawei devices that don't have FCM like the Huawei P40 that are sold worldwide. This isn't a niche issue -- Huawei was recently the #1 manufacturer of smartphones outside of China, surpassing both Apple and Samsung. Although this ranking has fallen as consumers have become aware of the lack of Google Services on these devices.