Spectroscopic sampling rate of the 2740 clusters in Kirkpatrick et al. (2021) as a function of cluster redshift, colour-coded by the number of spectroscopic members. The completeness, S (Eq. (1)) is estimated relative to photometrically identified red-sequence members weighted by their membership probability. The latter is updated with the membership flag retrieved from spectroscopic observations and described in Kirkpatrick et al. (2021). These estimates are performed within a distance R_λ from the X-ray centre of each cluster (see the main text for details).