Under Colemak layouts, @antevens wrote:
One thing to note that many users tend to forget, left and right ctrl, alt and win/menu keys are not the same keys, they have different scan codes and mean different things to the OS, this is especially true for international layouts but also affects a lot of other things.
This seems relevant here. I mostly use macOS, which does not make this distinction — however, I’ve found that mirroring all of my chording keys on my Kinesis Advantage keyboards has significantly reduced hand pain. (Shift, alt/option, control, and command are in an arc for each thumb, so I can use right-thumb modifiers for keys struck by my left hand, and vice-versa.)
So perhaps there are arguments for including left and right versions of these keys for all OSes.