This is quite stupid in my opinion. I don't see how you "help" the language discouraging others from forking/improving the project.

While I agree, I can also see where the creator is coming from here. He is working on making his very unfinished language, and someone else comes and starts making an optimized LLVM version of it. The creator certainly doesn't want the language to always look like it does today—after all, he is still actively working on it, adding new features every week. But he is afraid that the LLVM version will be so much better that people will use that instead of his work-in-progress compiler, and so they will be stuck with an old version of his language.

I think he should have handled it better. Maybe explaining why he didn't want a fork (he may have done this, I haven't watched his porth series specifically), and maybe asking the guy to put out something in his README.

Freezing the public version while continuing to work on it in private but not allowing anyone to see what you're working on... doesn't seem to me less likely to create a fork...

Either people are going to stop paying attention to it entirely (is that a desired outcome?), or they're going to mess with what's public, right?

I don’t agree at all. Unless the author enforces it with license or some legal means and/or the LLVM version author was rude, this is how competition works in a good way. And to be fair, performance is not the only reason people pick between different implementations. A good example being CPython: there are plenty of JIT-enabled python in the wild but I don’t see people turn away from CPython

This is what can happen when you design and implement things in the open. And it’s completely fair for it to happen as long as the design is open source. (Can it even be closed source…?)

But the author responded by temporarily closing it to the public. Which is also OK. He clearly didn’t want to deal with the consequences of openness.

I don't think you can say "completely fair" just because it's legal. Fairness invokes morality which is often something stricter than the law.

Yes. I guess I am talking about moral fairness as well.

When you design something in the open then it is... out in the open. That’s the default in our software culture.