Traveling from Mexico to the US is restricted by this notification issued by the DHS and continually extended every month. However I can’t seem to find an exact law restriction land border travel to Mexico from the US. The Mexican consulate in the US merely re-iterates the US version of the restriction, even if you switch to the Spanish version of their website. Every other website that I can find likewise merely refers to the US regulations, not the Mexican regulations. I can also find news of the Mexican state of Sonora closing the border in July 2020 but this seems like a temporary regulation that only applied to one state.
Hence… does this law or regulation actually exist on the Mexican side? In practice the land border is definitely open but I prefer to follow all rules and regulations, so I won’t be driving into Mexico unless I’m certain that its legal to do so. Going back to the US won’t be a problem as I fall under one of the exceptions so I just need to make sure I won’t be in violation of any Mexican regulations.