GTA 5, unfortunately, falls short in terms of realism, especially when it comes to vehicle design. It wasn't primarily created with a focus on RP. In the game, you'll come across numerous vehicles that can be considered unfit for road use, lacking essential equipment and features, such as a physical plate, side mirrors, etc.
Should we say that plates cannot be scanned, and that the model should be seen as it is, it will significantly impact the overall gameplay experience, as players might find themselves constantly pulled over, subjected to vehicle searches, and having their vehicles impounded due to the impracticality of their vehicles, a choice that neither they nor we made.
To address this issue, especially concerning many motorcycles, we've said that all vehicles in the game have license plates. The script also assigns a unique license plate to every player-owned vehicle.
The only cars that do not have a plate that will come back to you as a player are scrapyard vehicles, stolen NPC vehicles, or simply, vehicles of other players. You may want to explore one of those options if you intend on engaging in criminal activity and don't want the vehicle you're driving, to come back to you.