It is actually now possible to remove a vdev. It's a new feature, not sure if it works for all RAID types, but I just removed a mirror from my pool without any

It is actually now possible to remove a vdev. It's a new feature, not sure if it works for all RAID types, but I just removed a mirror from my pool without any issue. It migrated data off that vdev and into the rest of the pool just like a resilver.