> etymology should be tagged on all the named features where it applies and being tagged on the relation doesn't negate that

This is not evident, nor does the wiki page say so. Yes, the "name" itself is added to every individual way, as name may vary along the route, different names may apply in relation to different routes, and so renderers generally require a preferred name. But why would you repeat etymology data for ever road segment? Route relations exist mainly to allow better organization of the data, so that it wasn't necessary to dump the data about all larger objects that a way is part of into this individual way.