According to this, no. Notification data is stored in cleartext on FCM servers by default, but data can be encrypted with a 3rd party library.
data field in a push notification is optional, so no big deal if things aren't encrypted if no data is added.
Signal, for example, just makes a new notification with an FCM ID, a notification type, and the notification's priority. The
data field is empty for messages. When a notification comes in, the app launches in the background, pulls down new messages, then the app displays the notifications itself. All Google can see is the user received a Signal notification.
I don't know how other apps do it. I wouldn't be surprised if many apps that use push notifications store data in cleartext for their own convenience.