Stellar mass – total stellar metallicity for simulated and observed galaxies. Symbols and colors are as in Fig. 5. A shift of −0.2 dex was applied to the stellar metallicities of simulated subhalos following Nelson et al. (2019). The dashed black line indicates the median stellar metallicity of quiescent subhalos in TNG50. Shaded regions correspond to the local stellar metallicity relations from Gallazzi et al. (2005); the solid black line follows Gallazzi et al. (2005) for z = 0 and z = 0.3, and Gallazzi et al. (2014) for z = 0.7. Observed relics are more metal-rich than other compact galaxies, deviating from the local mass–metallicity relation, while simulated compact galaxies are consistently more metal-rich than the quiescent population, independent of their SFHs.