These are not delivered along with the notification-- they're provided (client-side) by the application that the notification was sent for. So, while a notification could display dynamic content here depending on the content of the notification (for example, this is the same mechanism as used by Mail on iOS for mail previews) and the preview code should take the untrusted nature of the content into account if it does, it doesn't have to and it's not any different than running the application itself in terms of security issues.

Not sure how macOS does it but iOS has notification service and content extensions. The original notification in itself is pretty lightweight. However, the service extension allows an app to perform additional downloading and data retrieval from which it can augment the notification data. From there, the notification content extension can take this enlarged notification and present a custom, interactive view controller. So yeah, almost none of the data is from the notification itself. I haven’t explored the limits of what APIs are available to a notification content extension but it wouldn’t surprise me if what’s seen here is possible on iOS, too.