Kata

I agree with @FArekkusu, if there is only one possible solution for a problem (espacially Code-golf), It just isn't that much fun. But nonethelss I don't consider it an issue. And no there is only one possible solution and not 3.

There are two possible solutions and not one (EDIT : make it three)

We must be living in different worlds, because all I see is the same solution with the difference as big as slice an array VS slice a string, and get 2 chars by index VS get 2 chars by slicing.

Well, atleast people'll learn something

Code Golf means find the shortest possible solution, not "find the shortest solution, but it has to be performant, and can't use this, and can't use that, and can't use...". It seems you're seeing yourself as a "teacher" if you think people are "learning" something, but they are not. Will I learn something by solving a well-structured, well-made kata imposing an interesting challenge? Well, maybe. Will I learn something from golfing the solution to a super easy kata into unreadable mess with dumb restrictions? Pffft, not at all.

Codegolf that only has one solution is fine to me.

The real problem is, it is counter-productive to write a problem with black-list approaches (aka a big ban list of things that "deviates possible solutions from the intended answer"): it's a very bad way to enforce intended solution and will make everyone hate you for making them have to guess what you're smoking when you're writing the problem. Just look at all those Interview 2.0 questions and the universal hate everyone has on them. (Also they've been proven to be ineffective, so...)

If people can find lots of alternate solutions to your problem, you need to redesign the problem itself so the intended solution is either naturally enforced, or is the path of least resistence. Adding more things to ban list is just applying lots of duct tape to a poorly designed problem: it's ugly, and it's not gonna hold.

Yes, it's kind of thing like You came up with an idea, found the shortest solution and you add all the restrictions there must be to force people write the same code as you wrote. That's how russian education system works. People don't think the same way...