Apple has many, many tools to prevent this from being a problem. To name a few:

- They can lock the API behind entitlements that must be manually requested and reviewed by Apple

- They can lock the API behind a user approval

- They can regularly prompt the user if they still wish to share notification data with the watch (same way they do to keep giving an app localisation data)

- They could enforce that the notification is sent encrypted over bluetooth, without giving access to it to the app itself - forcing its handling to be done on-device - if they're so concerned about this.

It's also worth noting that this problem is a complete non-issue for Android, and nobody seems to believe it is an issue there...