Unfortunately this is "the lesser of two evils" for the meantime.
The game has a lot of workarounds to help mitigate this such as:
- Maps can opt-in to "underground volumes" that mark areas the player is allowed to go underground, and teleport them back up if they are somewhere they shouldn't be.
- The game does its own capsule sweep after Unity's character movement sweep to check whether the player went through a wall. This is far from perfect because it uses a smaller volume to reduce false positives during substeps like when the character falls off a ledge or slides against a wall.
- Particularly exploitable spots have "kill volumes" to discourage out-of-bounds behavior.
- Shooting back into the map through one-way walls doesn't deal damage. It shows a hitmarker on the client, but damage is not applied on the server because it needs to be visible both ways.
The potential future improvement would be enabling CharacterController.enableOverlapRecovery to push the player back out of walls, but its current problems are worse:
- It can easily push through inward slopes like the walls of the barracks at PEI military base or cliffs that bend outward. Like, it is laughably easy to run straight through certain walls with this option.
- Since vehicle physics are simulated by the driver the car can easily overlap the character in multiplayer. In testing this was exploited to push players through walls of bases. When(/if) vehicle physics are made server authoritative in the future this would no longer be a problem because character movement is server authoritative as well.