I think one of the main aspects of this is that no core extensions actually use the "Approval" state of a post, the Approval extension is the only extension that both defines and use that value.
I believe most core extension features are based on gate and visibility, so if that's the case you would likely get notifications for any post you are able to see, which means moderators and admins might receive notifications for such posts while regular users won't.
There are also a few features that don't use the gate or visibility at all, in this case it's possible that the Approval state be entirely ignored.
I haven't looked at the source code of the Subscriptions extension recently, but I don't think it has any code specific to Approval.
It's important to make the distinction between the different kind of notifications, because each comes from a different extension. The code in Subscriptions extension and in Mentions extension might have some differences in determining which users should receive the email. There also other notifications defined by community extensions.
