Redshift evolution of the spectroscopic β from z = 4 to z = 14. The best-fit linear relation of all the individual galaxies (gray points) is shown with a red solid line, while the best-fit to the medium points in the redshift bins (red circles) is displayed with a red dashed line. Average UV slopes from the literature are also shown for comparison. The blue points are the XBGs studied in Sect. 4.