That is just how it is implemented in one map project. You have to look at our OSM documentation, because that is what other map projects will use as a reference:
https://wiki.openstreetmap.org/wiki/Key:osmc:symbol

"waycolor - Name of the color in which the route path itself should be painted."

And yes there are maps that use it:
* https://osmapp.org/
* https://mapy.com
* OSMAnd app
* https://www.wanderreitkarte.de/
etc.