Out of all of Sonic the Hedgehog's ccute, colorful sidekicks, there aren't many as widely popular as Shadow. The blue blur's evil counterpoint turned ally has been a popular pick for best character among the Sonic community upon his introduction to the series in Sonic Adventure 2, though you wouldn't really think so considering how often he's left on the sidelines. For example, Shadow didn't feature in Sonic Frontiers, and it was revealed by Sonic Team's Yakashi Iizuka that he won't be appear in Sonic Superstars either.
In an interview with Shacknews, Iizuka revealed more details on Sonic Superstars and the series in general, explaining that Shadow stans will have to look forward to the third Sonic the Hedgehog movie for his next official appearance. However, he did throw out a lifeline to those still hoping to see him in another game, as Iizuka explains that he'd like to include Shadow in a future game at some point.
"Unfortunately, Shadow's not in the game, sorry all you Shadow fans," laughs Iizuka. "This is a classic series game so do understand that Shadow is not a part of the classic series, but do be hopeful for Sonic 3. I think for those that saw Sonic 2, you saw a nice reveal after the credits. Do stay tuned for Sonic 3, and do know that we do want to create a game with Shadow in it for all of our fans in the future."
You can understand the frustration from Shadow fans about his lack of appearances in recent games, especially since he's now showing up in Sonic Prime and the Sonic the Hedgehog movies, but it sounds like fans won't have to wait too long to see him return. Whether that be as a playable character or just some inclusion in a game's story is unknown, but at least we know Sonic Team and Iizuka haven't forgotten about him entirely.
Elsewhere in the interview, Iizuka also explains that modern and classic Sonic games will be developed seperately for the forseeable future, explaining that these "subseries" will have their own resources and projects going forward. Previous games like Sonic Generations and Sonic Forces mixed up the two subseries rather effectively, although it sounds like the modern and classic Sonic styles will be kept seperate from here on out.
When it comes to currently unannounced titles, Iizuka was also asked whether we're likely to see a remake of Sonic Adventure 2 any time soon. While he seemed to enjoy the question a fair amount, he does state that he and Sonic Team want to focus on creating new experiences right now rather than focusing on older ones.