Are they?

For a texture to be usable in online gameplay, there must not be more than 50% invisibility of areas that show the form of the tori. If it hides the natural shape of your character, it's probably not good. And yes, more than 50% invisible textures are not allowed and thus warnable/bannable. Invisibility is alright in small amounts

Kay

Why can you get banned from this?

If a person has 100% transparent (or close to it) textures, it would be hard for the other player to discern where the body parts of their opponent are and thus making it unfair competition. Since the opposing player cannot see the actual moving ghost of the player, it becomes a disadvantage.

If you are caught, most cases you will be asked to stop before you get banned. Just make sure you can actually see the body part at least enough to notice it's there and you will be fine.

i got a question for this in same time,can you do full invisible if there only friend in a private game?

No problem if your just showing off. Don't play alot of stuff or for to long with it.