Well, C-H noticed over a decade ago that, for whatever reason, the conductor sizes <1mm^2 don't follow the same Renard series used for 1mm^2 through 25mm^2.

(For the bigger wires, I can understand transitioning to finer steps, to enable more precise sizing and minimize wastage; but I'm not sure about 0.5mm^2 and 0.75mm^2.)

If we extend the Renard series below 1mm^2, that gives 0.63mm^2 -- or 0.6mm^2 if rounded off, like with 1.5mm^2 and 6mm^2.
0.63mm^2 flexible conductors could be assembled from 20/0.2, or 35/0.15 (I definitely prefer the finer strands, especially in H03VV-F and H03VVH2-F); if rounded off to 0.6mm^2, then 19/0.2 or 34/0.15 would also work.

This just happens to be very close to an imperial size (to BS 2004) used by old flex in the UK (and Australia, going from memory) -- which is nominally 0.001 square inches (0.67mm^2), made from 23 strands of 0.0076" (about 0.193mm) diameter.

I believe some 0.25mm^2 flex (constructed as 14/0.15) also exists (or did in the past), and was used on a few low-power devices in the UK (with a 1A plug fuse); presumably, it's been ditched for breaking too easily. However, it could still fit into the Renard series, if you add 0.4mm^2 (which could be made from 23/0.15) along with 0.6mm^2.

Anyway, to get the all-too-obvious question out of the way:
If 0.5mm^2 flex is allowed by the relevant standards, why is it so little-used? (Mostly, it seems to be on either appliances that demand a highly flexible cord, e.g. electric blankets; or else very cheap-and-nasty items, including a few low-end fans I've seen.)
Perhaps because, for tensile strength and fault currents, it's a bit borderline...