That's not true. Both iOS and Android support sending encrypted notifications that are decrypted on-device.

How do you think Signal notifications work?

Note that Signal only uses push notifications to wake the app up. Then it directly connects to the Signal service to receive messages.

We are talking about whatsapp, no?

It's strange that your comment is not on the top.

I looked at the whitepaper and they didn't even mention it.

The reason e2e encryption got enabled is to make people feel safe.