I've never seen one that small either, 1/8" is the smallest I have. My go-to supplier goes down to 2.5mm, 3/32". But they do have this interesting new "arris rounding" bit down to 1.5mm:
https://www.wealdentool.com/acatalog/Online_Catalogue_Arris_Rounding_964.html
Like a round-over, but with a very slight chamfer on either side of the curve so that the corner of the bit can't dig in and leave a line. I'm guessing it's aimed at production scenarios where it would save time compared to sanding.