That's what https://en.wikipedia.org/wiki/Ezboard was where your username worked across all forums.

https://www.proboards.com/ is still around but doesn't have global accounts as far as I can remember.

I'm not convinced there's much payoff waiting for you if you were to create such a system.

I think that Reddit's format is so popular because you get to just scroll a stream of provocative/vetted headlines. Even the comments are geared in a way where you're scrolling a stream of one-off provocative comments. There is no long form conversation over time. Everything is ephemeral.

By the way, kind of interesting: https://en.wikipedia.org/wiki/Ezboard#Technology