Unless I am mistaken, keyd is a kernel level input remapper (like kmonad and kanata). So it is a lower level component: even if you use keyd, xkb will still be there, downstream. The situation would be roughly the same as with xmodmap... except it won't even be enough!

Indeed, this method consists in mapping scan codes to other scan codes. At this level, there is not the notion that a key produces a character yet. Hence concretely, kernel level input remapping (like QMK, btw) cannot be used to map a key to a character that is not already declared in a higher level component (usually in xkb or xcompose, but it can also be a another component using X11 API, such as xmodmap, for instance).