As per subject. Is this possible at all?
The WWDC video only talks about the communication / push notifications. There doesn't seem to be any update to UNNotificationContent to allow setting of attributed strings.
If your app makes use of communication notifications you can even include Genmoji and other image glyphs in your notifications with the new "UNNotificationAttributed MessageContext API". For push notifications, the payload just needs to contain a rich text representation that may contain image glyphs.
We recommend that you use a Notification Service Extension to parse the rich text, download assets, create the attributed body and update the notification content