I was an original Thunderbird pre-1.0 (from 2003) user and prior to that, Netscape Mail, and am quite certain it has had bayesian spam filtering all this time, at least since the late ‘90s. That was a headline feature in the early days. My first email account used POP3 through a shared web host for my own domain in that era.

Edit: Yes it’s still there https://support.mozilla.org/en-US/kb/thunderbird-and-junk-sp...

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.