I understand...
Integrations with external services are always very complex, and sometimes need specific agreements with big companies I can't handle.
I have spent a lot of time (and it's never finished) to connect to Strava. Each new activity type is complex to add, and documentation is very poor; it requires a lot of reverse engineering...
I have several requests to get workout plans from external services, but it's always a different service (intervals.icu, novio, final surge, training peaks…).
And my app has to be ready to accept plans with targets, alerts, and settings that can be more complex than what I have built...
So it's not excluded; it's in my to-do list, but it's a huge task that probably needs to be refactored completely this part of the app.
Unfortunately, Apple has created an API for that (Workout API), but it's only for creating plans outside the Exercise.app and using them in their app. It's not possible to use it for other workout apps like mine.