Note that in this cutscene, he clearly falls through a Chaos Control warp. I suspect they put that effect in there as a “back door” if they ever wanted to bring him back. And they did.

@Princess♠Molestia  
…when they found out Shadow didn’t actually died.

@Beau Skunky  
..Rivals 2, dude. He teamed up with Metal Sonic in his own story arc.
 
And the reason that Sonic Team’s run out of things to do with Shadow now is precisely why I dislike the Shadow game. They threw the character’s main traits out the window for the express purpose of pulling a rather poorly-told ‘amnesiac origin story’.
 
Let’s put it this way: In the entire 20+ years of Sonic’s history of gaming, have we ever once heard a single word about his past? No. We didn’t need anything for Shadow outside of what SA2 provided us with either, at that. And make no mistake, I don’t just take issue with the Shadow game - it’s also Sonic Heroes’ fault, and more prominently, the fanbase’s fault as well as Sonic Team for caving to the fanbase’s wants. They shoulda left Shadow well enough alone after SA2, and then have him reappear years later as part of a new story.
 
No weirdass alien backstory that makes him even MORE like a Vegeta-clone, no emo-angsting about ‘WHO AM I????’ or amnesia nonsense (seriously, between SA2, Heroes and ShTH, the dude has the memory of a freaking goldfish). Just focus on actually building a solid character around Shadow. But at this point it’s too late, the damage is done.
 
That’s the problem I have with it. Wasted potential.

@Background Pony  
I don’t know, I enjoyed the Shadow game regardless of it’s flaws, and hate it gets. It does seem like Sega’s run out of things to do with Shadow now. He hasn’t really been in a Sonic game sense Sonic’06 except for cameos. (Like in the racing/olympics games.)

@Beau Skunky  
He didn’t win, no. Sonic still came in first. And I’m an old Sonic fan, and I don’t hate Shadow - I just think they used him poorly after SA2 and onward.

@Princess♠Molestia  
Well, while most older Sonic fans hate Shadow, he actually is quite popular with the younger ones. I think on the official fan-favorite Sonic character poll, Shadow won, didn’t he?
 
As for me, he’s not my favorite Sonic character, but I don’t hate him either.

I think most people here would have this reaction instead.