What Could've Happened To The Classic Era Characters During the Modern Era?

[SticksSuperFan14]

While the cast from the classic era's disappearance used to be explained by modern and classic being separate dimensions, they have now been retconned into being the same timeline. This begs the question: What happened to them by the time the modern era began? I'm curious about any headcannons you guys may have about where they may have gone off to during this time.