Compactness–temperature diagram translated into directly observable quantities and high-energy cutoff distribution as a function of redshift. Left: X-ray luminosity vs. high-energy cutoff, updated from L19 (see main text and Tables A.1–A.3 for details). NuSTAR measurements are in green, non-focusing telescopes’ in grey, high-z AGN from L19 in cyan, and our measurement in red. Magenta downward triangles mark the averaged values for BASS AGN from Ricci et al. (2018), while the magenta upward triangle marks the median point of Seyfert 1 galaxies from Akylas & Georgantopoulos (2021). Yellow (orange) areas delimited by a thin (thick) line show the runaway pair-production region for a 10⁸ M_⊙ SMBH in the case of slab (hemisphere) geometry. Dashed (dashed-dotted) lines mark the same thresholds for a 10⁹ M_⊙ (10¹⁰ M_⊙) SMBH. Right: high-energy cutoff vs. redshift plane for the same samples as in the left panel.