Samsung's Galaxy Note 20 Ultra features is the first to feature Samsung Display's 'Adaptive Frequency' technology. The company's display division revealed the details as it moved to make the tech available to other OEMs.
https://www.notebookcheck.net/The-Galaxy-Note-20-Ultra-display-is-the-first-to-use-Samsung-s-new-variable-rate-refresh-tech.485237.0.html
Where else has this been used before? I can only recall IGZO, which only goes up to 60Hz, and in practice the lowest refresh rate would be ~15Hz, IIRC.
In mobile devices I've only seen it used in the iPad Pro models, but even those can only go down to 24hz. The Apple Watch can go down to 1hz when AOD is enabled and active.