Present in specialist clients like Eudora, perhaps, but I think for mass-market users that only touched webmail and Outlook the introduction of threading in Gmail must have seemed heavenly.

I never said Gmail invented threaded emails, just that it brought threaded emails to the masses. Outlook and almost all webmail didn't have threading support at the time.

Opera's email client had them too, I know I was using that around 2000. It was the first time I'd seen the feature, and it was one of the major reasons I used Opera for as long as I did. But Opera, then as now, was far from mainstream...

Tin. I used tin from mid 90s until around 2005, and it supported threads.