Languages should not be designed around the assumption that people use an IDE.

It can. But seeing the text there is faster than having to hover to see what the type is.

It exists. It’s fine. People obviously like it.

Some don’t, I’m one of them. I don’t see the advantage is very big at all. I don’t think it’s worth the trouble.

But that’s me.

What if I'm just looking at a pull request on GitHub? Sure I can check out the branch etc but that's just adding more friction.

Bingo.

Sometimes I doubt most hacker news commentors have ever worked in big corpo environments where you have to regularly review large PR in some broken webapp like GitHub.

If you haven’t used GitHub plugins in your ide to review you should really give it a try. It’s really night and day.