I can't recall the name, but I vaguely remember a Bayesian spam filter for arbitrary POP3 accounts in the 2000s that had a local web frontend, and how excited I was at its effectiveness.

I believe that the shift from "my one computer" to multiple clients (computer + phone + webmail) probably has something to do with it. Even with IMAP sharing state, you still don't have a great way to see and control the filtering, except by moving things in/out of spam folders.