And where is this documented or discussed? 
On the contrary, markings were stilll  discussed around that proposal https://community.openstreetmap.org/t/large-scale-change-of-traffic-sign-to-traffic-sign-id/107508/61 
`=GB:1003A` doesn't have that many more `traffic_sign=` than `road_marking=` . It was seemingly added by someone for self-docuemntation. That still doesn't explicitly show it is marked but not signposted. It may be incomplete data.  https://wiki.openstreetmap.org/w/index.php?title=Tag:highway=give_way&oldid=1641607
Instead, `traffic_sign=` has 10k `=none` and ~800 `=no` . These are more likely to mean there is no plates, rather than no traffic control devices including markings. 