Also found at this issue. Posting here in case I get a reply sooner.

The value 0.03928 (as used in what was formerly a note to the "relative luminance" definition but what is now a "normative" part of that definition) comes from the sRGB proposal. This is outdated and either 0.04045 or 12.92 * 0.0031308 = 0.040449936 (I don't know which is used in the IEC standard) should appear in its place.

I understand, however, that all three thresholds don't make a difference in the results if only 8-bit linear sRGB values are involved. On the other hand, if no change to the 0.03928 threshold is planned soon, a note on the origin of that threshold should be added to the document (in particular, why that threshold was chosen rather than the one given or implied in the IEC standard).

EDIT (Mar. 27): Clarification.
EDIT (Apr. 9): Struck out part of the text. Since so few source code files on GitHub use 0.040449936 compared to 0.04045, I reasonably believe that the IEC standard uses 0.04045 as the relevant threshold, assuming it even mentions an inverse transfer function.