Room creation setting to limit the maximum amount of mice/players that can be in a room. Players often use /pw to limit the amount of mice, but that locks the room entirely so it cannot be re-filled when mice leave.
Many players who resort to using /pw to limit the amount of mice in a room are using a very slap-shod way of doing so, yet they will still use such a thing unless actual functionality was added to room creation.
Players will do such things anyways regardless if this functionality was added, so it may not make much of a difference to release it.