Interestingly, argon as electric propulsion propellant has been used since the 60s (see Wikipedia's non-exhaustive list:
List of spacecraft with electric propulsion - Wikipedia
However, the claim that no Hall-effect thruster has ever used argon *in space* appears to hold water. Nevertheless, and notwithstanding SpaceX's stated performance for these new Starlink v2 thrusters, Ar was routinely, seamlessly and interchangably used with more precious noble gases, such as the industry-standard Xe and the Starlink v1 (and others) Kr, for ground testing. Translation to space-bound Hall thrusters was not generally undertaken because of its comparatively worse performance compared to the former: in simple terms, being a much lighter and smaller atom, it requires more energy to ionize and -weaker effect- provides less thrust.
There is quite a rich literature of recent (from the late 90s) revisiting of Ar as an in-space electric propellant though, especially in mixtures with low amounts of Xe, which provide some interesting advantages in performance beyond pure cost: from a cursory reading, it appears collective effects of xenon atoms banging into an argon environment can greatly increase the exhaust's kinetic energy (while perhaps relatively increasing erosion). Of course, there is a pervasive industry-wise xenon scarcity that is affecting all manufacturers, so there's that facet to take into account too.