As for this, I suggest to have a script running that detects when a player has logged out or crashed. It leaves the vehicle in the road for 10 minutes before it respawns it at it's law /savecar spot; that'll negate the littering.
Wow, I thought I replied to this. Player Owned Vehicles should only be spawned in when the player is online and on that character. Otherwise, people can damage and mess with your stuff while you are off line and have no chance to defend your property.
We may do a trial run of this at some point in the future however it is not a major priority and would require a decent refactor of how vehicles currently spawn/save. Also as Bruh said AltV still has this weird vehicle rotation issue (I think its when people stream the car in and out it rotates a small amount) as you can notice with the FD/PD vehicles on the main server. We would definitely have to wait for that issue to be fixed before attempting this trial.