Depending on how you look at it, there may be two distinct parts to this:

a) API to not just read notifications but also perform the notification quick actions (if any), e.g snooze for a calendar event, mark complete for a reminder, and of course reply for a text (SMS or otherwise). This seems entirely reasonable and ludicrous that it doesn't exist.

b) API to access SMS / Messages. That one appears to be heavily guarded because security / E2E (for iMessage).

I mention b) because a lot of times people invoke the problem a being b) (and possibly a problem in its own right, forcing one to use Messages for SMS) but really for watches a) is sufficient and probably much more relevant.

There's also a.1) API access to media (images) in notifications.

In any case, DMA could definitely help crack both.