im almost certain thats what they are. like the rules for side mirrors cover the view angle and positioning and such, so for instance a drivers side mirror cannot be concave or convex it must be a flat mirror but passenger side does not (which is why it has the warning about objects appearing closer than they appear), but i dont think that it ever states they need to be unobstructed

this is frustrating for motorcyclists customizing their bikes because (at least this used to be the case) those little round mirrors people would put on the end of their handle bars to replace the bunny ear mirrors on their motorcycles are technically illegal. even though they often function better than the legal ones because they give a better rear view by their nature (and no good motorcyclist would ever merge lanes without turning a little and looking first anyways)