Surely you can work that into the layout or make it less distracting. Look at how the other MvM maps are detailed.
It has to do with an other reason there:
MvM is a fps heavy mode already. And for that you need to optimize whatever you can. The out of bounds areas for that reason are the first thing to remove detail on. To hide that low detail many stuff on the side is higher so its barely visible it has that low detail. Using other methods often will make the map heavier.
For other maps the detail level in the skybox can be higher. On normal maps having a higher focus on what an area has to show is more important. If you look at some maps you can see that the playable area still shows like its going on, except it has a huge gate in front of it. That gate is a clear barrier and blocks the detailed area that is behind it.
When you detail an area to a higher state, the barrier should be more clear aswel. If its a flat displacement with only a road texture on top of it, people will understand its not playable. As the area itself just doesnt look playable.
However, once it becomes more detailed the gap between playable and unplayable needs to become very visible. This can be done fences, it can be done by clear road blocking props, it can be done by a deathpit, and it can be done by simply making it impossible to go there to begin with (high flat building or cliff side with a few holes that have detail in them yet are clipped off).
The barrier has to make sense and the detail behind it should clearly indicate its not ment to play on.