I am also wanting/needing R12 export functionality directly from AI. Right now our only option is to use AutoCAD or SolidWorks and import a DXF from AI to one those program and export again. My version of CorelDraw doesn't have R12 export capability but that might be another option with a newer version, can't say for sure though. Check before you buy.
Thanks for reaching out. I am afraid exporting to Dxf R12 is not possible in any of the Illustrator CC versions of Illustrator. I checked all the versions from 2013 to 2021. I even checked in CS6 with the help of a colleague at work who had it on his old machine, and the Dxf r12 option was missing.
Also, R12 DXF was released in the early 90s, which is why export options have been phased out. Probably in CS5 or earlier, DXF R12 export options will be available, but those versions of Illustrator are no longer available for purchase & download.