Won't ever happen. It'd be both a financial & PR disaster - other than pleasing a few angry pisspots, nobody would gain anything from it.
First off, all the work that was put into that champion - designing them, balancing, programming, testing, Q&A, etc - would be completely wasted. The playerbase of that champion would be supremely pissed, and it's not a good message toward the guys who made the champion either.
They also can't just delete content that was paid for. Aside from any potential legal issues, the amount of player backlash would be immense - People would be hesitant to spend RP for multiple reasons, and the bad publicity would just really hurt them in the long run.
So they'd have to reimburse everyone who has bought the champion and any of his skins, which would cost them a metric fuckton of money - and setting that up would probably also require a lot of work, since you can't just send out X refund tokens to everyone.
And it's not like they need to; if they really don't want a champion in the game, they can just nerf him into unplayability. They did so in the past, more than once; one of the biggest examples is the old Eve, who was allowed to be shit and trollpick material for years. Still a scummy move, but much more subtle; the only real downside is angering the playerbase of that champion, but otherwise it's nowhere as bad. And people will still buy them & their skins occasionally, and definitely after a rework.
The only time they would do this probably would be immediately after release; and both Riots internal testing & the PBE would have to drop the ball really, really, reaaaaaally bad. Azir's release was problematic, but to warrant this, it would have to be 100x worse.