Saw as early as 99' for the Impreza, but unsure of when the Legacy started either using them, or offering as an option? Are they all behind the glove box?

Check cars101.com. I'm not sure either but I don't think the legacy had them until 05. Cars101.com may have it listed in the equipment specs, but you'll have to check each year individually. Look in the archive section.

I'm gonna say 01 - but actual legacy models could be different than Outback ?

 

i THINK some late 90s Imprezas could be retro-fitted for them - I think there's a youtube video or a pictorial guide i saw once so,,,I wouldn't discount the possibility of making a filter fit 'somehow'.

OK, I'll assume we don't have one then. Was thinking if it did have one, it'd probably be extremely moldy and filthy by now. I don't seem to get very much if any dust build up in the cabin on the dash, so thinking it might have been possible. Next time I'm at Auto Zone, I'll check if anything was available this far back.

2000 is the first body style that had cabin filters, though not all of them had it. 

In some of them the space is there, just need the filter - that varies by year/model.

Slightly off topic but the 00-04 legacy cabin filters are pointless because they leave a 2"x4" bypass hole past the filters because of where the blower motor resistor is. So the filters may catch some stuff, but plenty can get past the bypass. I was disappointed with the first and only pair I bought and installed.