Apple has already updated the documentation. The 2KB for the maximum payload size is just for devices running iOS 8. The official doc says:
In iOS 8 and later, the maximum size allowed for a notification
payload is 2 kilobytes; Apple Push Notification service refuses any
notification that exceeds this limit. (Prior to iOS 8 and in OS X, the
maximum payload size is 256 bytes.)
As per the updated Apple docs the size is 4KB.
- For regular remote notifications, the maximum size is 4KB (4096 bytes)
- For Voice over Internet Protocol (VoIP) notifications, the maximum size is 5KB (5120 bytes)
NOTE
If you are using the legacy APNs binary interface to send
notifications instead of an HTTP/2 request, the maximum payload size
is 2KB (2048 bytes)