Fraction of galaxies in different classes as function of total stellar mass. Blue, orange, and red lines denote the fraction of ageing, quenched, and retired galaxies, respectively. Solid and dashed black lines denote the fraction of galaxies with values of sSFR₈ and sSFR₉ below 10⁻¹¹ yr⁻¹. The grey-shaded region shows the fraction of galaxies with sSFR ≤ 10⁻¹¹ yr (estimated using nebular emission lines and optical photometry) from CC21a. Top and bottom panels make use of the posterior- and KDE-based classification (see Sects. 4.2 and 4.4), respectively. Left panels show the fractions computed using a Euclid sample of nearby galaxies (z ≤ 0.1), where the grey-hatched area denotes the mass-completeness limit (see Appendix A). The error bars were estimated from 1000 random samples, each selecting 20% of the total sample and computing the 5 and 95 percentiles. Right panels show the results from the IllustrisTNG sample at z = 0, including the true values (square symbols), where the grey-hatched area denotes the retired mass resolution limit (see Sect. 3.1).