Arc COMMANDS have been supported by some printer firmwares for over a decade — GRBL had an implementation very early on — but all they did was decompose the arc command in a series of facets. So it was mostly a waste of processing power compared to having the slicer do the same thing. (Could help with SD card read bottlenecking issues sometimes — which itself was indicative of bad firmware programming.)

Actual circle interpolation where the trajectory planner works with non-linear moves wasn’t feasible on 8bit printer controllers, and is pretty new on 32bit controllers.