This isn’t in the iOS app at all. I wouldn’t be surprised to learn that many users never visit the website.
They do offer manual route trimming from both ends, too. I've found this useful mostly when I forget to turn off my watch after running.
I wish they had a way to do an auto-trim so my bike rides didn’t include GPS bounce leaving the building. CommuteMarker.com had auto-tagging for Strava rides but not auto-trimming last time I looked.