It’s certainly in use, but I don’t know that it’s a good idea in this case. A network relation is a good idea when a network is discernible on the ground but individual routes are not. Calling the E road network a geographic entity is a little bit of a stretch. Taking the logic further would result in a continent-wide situation like we have in South Korea:
Taking it even further, someone has created a brand relation as an alternative to tagging brand=* and brand:wikidata=* on each shop in the chain. I’d have a hard time seeing how it’s a category whereas the E road relation is a geographic entity.
Most of the tags indicate the “name” of the network in various languages.[1] Most route networks have no single, formally set name, just a description, which is why we normally set network=* to mnemonics and abbreviations. Some maps like OSM Americana do have a use for the “names” of networks, but they can get this and other geography-adjacent information from Wikidata:
Some of the names, including in English, are incorrect. Someone expanded “E” to “European” out of a conviction that abbreviations don’t belong in the name, but as far as I can tell, UNECE always calls them E roads. ↩︎