Well, how things are is different than how we wanted things to be, is different than how we want things to be going forward. As far as I know, the symbol=* documentation was never particularly prescriptive about what counts as a symbol. That’s why U.S. mappers so frequently reached for this key when they wanted to associate a route with a shield. The osmc:symbol=* documentation may always have been stricter, but that didn’t prevent people from bending the rules and using it for shields too. The more recently approved trailblazed=symbols tag could have the same problem.
Most likely, this documentation was written by mappers from different parts of the world who are only familiar with the trailblazing practices in their respective countries, unaware of such significant global differences. So I’m glad we’re having this discussion. 
It all reminds me of the other kind of shield. If we establish a convention of tagging boundaries with their coats of arms, sooner or later we contrarian Americans will put our seals in there instead. I bet Central European mappers didn’t expect that we use these terms interchangeably:
If we want mappers to stop diluting symbol=* and osmc:symbol=* with shields, then there needs to be an alternative. At the same time, designing the perfect machine-readable shield language would be a distraction for OSM, and a less machine-readable shield key probably wouldn’t strike mappers’ fancy as the OSMC symbol renderers have for blazed trails.
To me, network tags are a suitable, if imperfect, mechanism for communicating the need for a particular shield design. Shield designs are reasonably frequently associated with entire networks in reality, so the ones that are specific to a particular route can be treated as exceptions. The display of shields to end users won’t be as automated and data-driven as for blazes via osmc:symbol=*. But as long as some renderer can display shields based on OSM data, I think the U.S. community will definitely be able to convince mappers here to leave shields out of symbol=* and osmc:symbol=*, just as Americana has been able to help the community rationalize highway classification and route tagging.