Just to be sure, a isn’t a “drop_line” a symbol of a water drop with a hollow interior and a contour of specified color? I fail to see how this is related to the symbols on the ground.
What would be the meaning of the relation in this case? The “merged marking” is a physical property of the way, while every relation can have its own “default” marking regardless, and the renderer should use the marking from the way if it’s available.
I think that systematically removing data should be done by a consensus. I produce for myslef maps with the Maperitive offline renderer, and I would like to have the routes simply colored, without shields. Till now, I did this by matching “color=" property. Now it’s broken. Not a big deal, but still… Who knows, maybe someone else was using this as well.
Regardless, it’s better IMHO to have "color=”, at least until we have some stable convention and we see that rendering is good.