No, it isn't. You can run a ZFS VDEV or a Linux mdraid as a single-disk RAID 1 unit until the remaining disk fails. You have an arbitary number of reboots/remounts to fix the problem.

So it turns out there's some nuance here. You can remount it multiple times, as long as you don't write to it, which I was familiar with. The moment you write again and don't explicitly remove the extra volume, you'll lose the ability to mount r/w, which is a pretty bad bug :(