>> 3d printers typically don't care about arcs or splines since they only use STL files which do not have "real" curves.
I would have thought the printer would support arcs even if the slicer doesn't. There is work going on to support arcs in slic3r (automatic detection of arcs) but it will be irrelevant for printers that don't support it. It's would also be possible for a slicer to import step files and find such surfaces. One can also approximate other curves with fewer gcodes if they use arcs vs lines - splines even fewer.
I assume most of the modern ones do but some of the older 3d printing firmware use a very butchered version of the gcode "standard" which eliminate or rename many of the unused codes.
I just flashed my Ender 3 using the latest Marlin version and I deactivated ARC support manually on it to save space. Considering how pretty much all 3D printers use a Marlin firmware, seems like ARC is supported pretty much everywhere.
I would have thought the printer would support arcs even if the slicer doesn't. There is work going on to support arcs in slic3r (automatic detection of arcs) but it will be irrelevant for printers that don't support it. It's would also be possible for a slicer to import step files and find such surfaces. One can also approximate other curves with fewer gcodes if they use arcs vs lines - splines even fewer.