[Bleuachdu]

The general rule of thumb for league and public play states that most if not all map glitches are cheats. What you are referring to in particular is called "going under a map" and can be done in numerous maps. This is most definitely illegal considering that the makers of the game did not intend for you to stand underground and shoot at unsuspecting victims. Some map glitches are, at least in my eyes, considered OK. For example, the rooftops in So. France are fine. They may not be where you'll look for players or where you'd expect to see them, however they are solid textured areas that you can get to with a little work. I hope this helps.