Well it is in...

https://github.com/D-Programming-GDC/GDC/commit/15907b95026d451f443b4b6d263c85e0524e517e


As for testing:
---
{
  @attribute("target", T)  void func(string T)() { }

  func!"sse.4.1";
}
---

Error: undefined identifier T.


Looks like the frontend rejects this - will find out whether this is a
bug or intentional...