Sonic Team boss, Takashi Iizuka, has spoken about the possibility of the ultra-edgy Shadow the Hedgehog appearing in the Classic series of Sonic the Hedgehog games. Iizuka told Game Informer in their most recent coverage of the forthcoming Sonic Superstars, that that while Shadow continues to be his favourite non-Sonic character, he won’t be appearing in the Classic series simply because its before his time and jokingly said “Unfortunately, Shadow was in a capsule at that time.” Here’s what he told Game Informer:
“Shadow is from the Modern series of characters and gameplay that we have introduced. The Classic series exists before the Modern series, so they haven’t met at that time. There’s no relationship, so we won’t be bringing Shadow into the Classic series.”

Makes sense, the classic forms are the younger versions of the characters, even 200 years in the future Shadow is still around and looks the same.
Of course Shadow wasn’t part of the Classic era because he didn’t appear in it and Sonic and friends never met him in that time. So, yeah. However, Sonic Adventure 2 was going to be his only appearance after they kill him off in the game, but base on fans feedback, they decided to bring him back to the series upon the release in Sonic Heroes in 2003.
This is what happens when people just don’t pay attention the only people in the classic timeline were, Sonic, Amy, Tails, Knuckles, Nack/Fang, Team Chaotix, Mighty Ray, Metal Sonic, Mecha Sonic, Eggman, Bark and Beak the Dynamite
Describing it as a different “timeline” actually opens up the doors for Shadow to appear one day. What you mean is that it’s in the past on the same timeline, so couldn’t happen before SA2.
He will never aopear in the classic time, because Shadow was made befire Sonic was even born. No one knows gow all the classic anthropomorphic beings were created, but one thing is sure, Shadow was in deep frozen sleep when all that happened. What I would know, is how zhe rest of all those anthropomorphic beings were created. Because it looks like only Cream jas a mother but none is known about the rest (and Archie comics as well es IDW cpmics are not canon.)
Makes sense I guess Shadow is a moden character and that’s fine