Jill was a character that kept being moved from one place to another to drive story for several leads. While this is exactly the kind of behavior one expects from a "supporting character" (helping drive story for others), her shift from one plot and/or motivation to several others may have contributed to her inability to be viewed as a major, long-term character. A lot of supporting characters like Ben, Peter, and Lilimae had one plotline or purpose, while Jill changed gears several times during her run. She started as a potential spoiler for Mack/Karen, then became (with Peter) a potential threat to Greg and his empire. Pairing her with Gary gave him something to do as his marriage to Abby fell apart, and at some point they decided to make her a spoiler in a potential Gary/Val reunion. Even in death she gave story to Val, Gary, Mack, Karen, and (to a lesser extent) Frank and Pat. She also went from a principled (if secretive) woman out to avenge her parents' deaths to a genuinely loving partner to Gary, then to an insanely jealous 'other woman' who plots to kill her rival and ultimately stages her own 'murder'. With so many facets to her tenure, it might be harder for viewers to grasp just how long she was on the show.
As for Austin's seeming lack of participation in reunions, etc., a lot of actors don't have a level of sentimentality about various roles they play. Once they leave a role, they don't look back and choose to look forward to what's next.