The DEC Alpha had SMT on their processor roadmap, but it was never implemented as their own engineers told the Compaq overlords that they could never compete with Intel.

"The 21464's origins began in the mid-1990s when computer scientist Joel Emer was inspired by Dean Tullsen's research into simultaneous multithreading (SMT) at the University of Washington."

https://en.wikipedia.org/wiki/Alpha_21464