If it was an OEM one, it was probably shot 5-6 years after the XJ left the factory.
Because people will spend money on them as an upgrade item. Which ones have you been looking at?
The reality is that you can run without a steering stabiliser without any major issues, but it depends on how much you like the feel of it or not. I prefer having one in there, but that's just me.
What prompted you to call it out? Is it weeping or is something else up with the front end?