From what i've seen in my friendslist on league, the people who paid for boosts haven't been banned yet, and they bought it a few years ago.
I doubt you'd get banned unless someone picks it up and makes a widespread announcement about it.
So low it's not even something you should worry about. If it were likely I'd have already been banned on my smurf accounts for completely anihilating silver/gold players.
I mean, I had 1 account boosted, ever, and it was banned for two weeks and lost the reward. I kept the rank though. All I got was 6 provisional games too.
I know quite a few people who have purchased boosts and no one has ever come cross any troubles. I would suggest not waiting until the end of the season to purchase a boost though, it seems like Riot likes to at least scare us around that time of year that they're on the "lookout for boosters".
The ban rate is VERY LOW. You should not be worried. I've had few friends of mine who boosted (I also got on one of their account and got him to gold) and they haven't been banned.