Not sure what you mean by “it never happened”. . . They built it but decided it wasn’t A14(M). Or they started building it but abandoned the project. Or it was proposed but never built.

In any case, maybe the life cycle prefix tagging scheme could be of use here. https://wiki.openstreetmap.org/wiki/Lifecycle_prefix

That would, for example, allow you to may the location of failed scheme as something like:

proposed:highway=primary

To the best of my knowledge, the life cycle tagging is not rendered on https://www.openstreetmap.org/ so it won’t clutter up the map for others. But specialized renderers may show it.