Yeah, "Classic" and "Modern" WERE separated and considered two timelines a few years ago but Sega changed their minds again. Sometime around last year.
New Sonic Mania Plus characters Mighty the Armadillo and Ray the Flying Squirrel won't be appearing in modern Sonic games, confirms SEGA during a SDCC 2018 panel.www.playstationlifestyle.net
The “Two Universes” Retcon is Dead: Sonic Mania is now in the same timeline as Modern Sonic & there’s only one timeline (Confirmed by Ian Flynn) Segahttps://youtu.be/K5k3_19I_Ws It’s a retcon to Sonic Forces, but one that I doubt many will complain about. But yeah, Mania now happens in Modern Sonic’s past. They talk about it at the three minute mark. Source: BumbleKing Videos (YouTube)www.resetera.com
...You know, until Sega changes their minds AGAIN, and Classic is revealed to actually be the FUTURE of Modern and not the other way around for some reason.
If that's the case, then I wonder where Fang, Mighty, and Ray are in the present timeline? Possibly killed off during an Eggman attack?