I would support that, but there are a couple concerns that I'd want addressed first. motorway=yes is currently used to mean motorway-like access restrictions, not physical characteristics. So, there would need to be a plan/proposal to migrate the existing data using this tag to some other key. The original proposal for motorway=yes actually was what we're talking about: https://wiki.openstreetmap.org/wiki/Proposal:Motorway_indication

The other thing is whether we should use motorway=* at all, or if there is some other key that would be better. I can't think of a better one at the moment; I think the proposal I linked above looks quite good. Of course, if we pick a key that isn't used now, we won't have to go through the process of moving the existing motorway=yes data.