Manual Sheathing Only is a mod that lets you play the game in a different way.

The first thing you'll notice when you install it is that the option you're looking for is no longer under the "Enable mods" section but is now grouped under "Manual sheathing only".

Now, when you activate it, you'll be able to play the game as you were before but you'll have to manually sheathe your weapon when you take it out of your weapon compartment. This means you'll have to switch the weapon manually from the sheathed to the unsheathed state.