As per the Firebase Notification docs:
Notification messages are always collapsible.
I was trying to figure out if they have some intrinsic collapse_key. And if so, does that count towards the collapse_key limit of 4 per device?
You are correct. All the notifications sent from the Web Console are collapsible.
Which means they have an implicit collapse_key
If a device has a pending-to-be-delivered messages from the Web Console,
this implicit collapse_key counts towards the limit of 4 per device.
来源：https://stackoverflow.com/questions/39655543/does-a-notification-message-count-towards-the-collapse-key-limit