Yes, you can turn the Slope Angle Map Layer on and off when you’re offline (and you don’t have to turn the Slope Angle Map Layer on before downloading an Offline Map—you can turn it on for the very first time offline if needed). The only thing to keep in mind when using the Slope Angle Map Layer offline is that you’ll only be able to see the slope gradient within the bounds of the area you choose to download for offline use.