It is bannable.
The issues is there is a lot of money involved so people will boost people. People want boosted due to small penis syndrome. Basically, they view their elo as having a reflection on their "alpha" status.
For example, one site has boost from silver 2 0 lp to gold 5 as $47. That's not much more than an "ultimate skin" and gives you shit talking abilities to "your friends". When you want up to higher it starts to involve hundreds of dollars and even over $1k.
I've actually met a booster in real life. He tried to pitch to me irl when I said I sucked. I told him I was gold and he's like that's not bad at all. He was telling me a lot of people lie about their elo to look better. He was also telling me a lot of players "think they x rating" but blame their teams and want "boosted" to their actual level. These are repeat customers due to them deranking b/c they can't compete at that level.