Saw this at a junkyard on the back of a first generation Outback wagon:

 

I have not seen a deflector like this before. Am guessing it is an after market product designed to deflect air flow across the glass to blow rain off of it.

They definitely had these 'dust' deflectors available as a Subaru Accessory for the 00-04 wagons (in addition to the spoiler). I would imagine they had them for the previous gen as well. I have no idea how to tell if it is OE or aftermarket though...

That's an OEM option on that gen outback. My family's 99 outback has that.

Yep, that's the same one my 1997 has. It came with a bug deflector on the hood as well, but that is long gone, leaving a lack of paint where it rested :-\