Rack 2 by Fek. Im normally not even into furry games, but it had a good foundation that had me interested. still holding out a microscopic sliver of hope that it's not dead forever, but yeah it's basically a lost cause. rack 2 reinvented revived the project for a while, but even they had to call it quits once it was obvious Fek wasn't doing anything with the game.
Radiant for sure -- mostly because I'm new'ish' to AVN's and it was one of the first ones I played before I even knew that 'abandoned' was such a big deal. So I didn't know it was just going to end and that was it forever lol -- like watching a TV show for the first season and it gets cancelled unexpectedly. Nowadays I set my expectations ahead of time when I play an abandoned game so it doesn't cut as deep lol
I don't think I have played many abandoned games (some are the type that updates once every blue moon at super slow paces with minimal content LOL) but off the top of my head, On Edge was one of my top games for a bit. Was really looking forward to the planned content and loved the art style, but alas it changed art styles before dying afaik RIP-