Is it possible to prepare for a mountain ultramarathon using Garmin's adaptive plan?

I don't think so. Garmin can't even estimate a remotely accurate finish time for a trail ultra. From all the evidence that I've seen, Garmin training features are completely oblivious to elevation gain or terrain. It just seems to assume marathon pace for any distances longer than marathon. 

In general Garmin plans provide basic guidance for events aimed at the average Jo. I think it’s always been the case that if you want to stray away from anything ‘vanilla’ you’re better off getting a coach.

No I do not think so. Even the race predictor couldn‘t handle elevation data - the time prediction was way off even after uploading the gps race courses. 
all the plans that came up for my 50k trail races were really generic marathon style runs - nothing with hills, elevation etc. 
I don’t think that has changed since my last race in may. 
Edit: can only advertise for this book: https://www.jasonkoop.com/book 

That's what I thought. Thank you for all the replies.