Optical depth to Thomson scattering, based on the extrapolation of the predicted ionisation histories from our CHE and GENEC models (see Fig. 8). The recent measurement by Planck (Planck Collaboration I 2020) is shown with the horizontal line, whose 1, 2, and 3σ scatters are denoted by the shaded regions. Again, the vertical lines show the redshift range (z ∼ 5.5 − 15) in which our predictions at high-z are extrapolated to the post-reionisation epoch.