Stryd is currently focused on 5k to Marathon training plans since training plans for these distances generally fit a large majority of runners. Ultra-distance plans require specialization to fit around your schedule & skills. Ultramarathons have different psychological and physiological implications that are best handled by a coach or the yourself if you are an experience runner.
The Stryd platform now accepts third party plans from TrainingPeaks, Final Surge, and 2PEAK, so you can buy or create a structured power-based ultra plan from one of these platforms and import it into Stryd.
If you have additional questions, contact firstname.lastname@example.org.