It is still a thing, but it's open source now instead of maintained by Microsoft. There was a release that finally supports Python 3 in December last year.
I don't know how useful it is really, if you really want performance then you probably shouldn't choose Python to begin with, or you use the libraries which may not be compatible with IronPython. These days it barely takes me longer to build a simple script in C# than in Python either.
It's so so.
Pythons core value is it's huge stack of lib's. And most important fall down with IP due to them using c and so on.
When we needed python c# interop it was better to use python.net and integrate that way. Annoying to setup but when it works you can get both to work seamlessly
I don't know how useful it is really, if you really want performance then you probably shouldn't choose Python to begin with, or you use the libraries which may not be compatible with IronPython. These days it barely takes me longer to build a simple script in C# than in Python either.