2026ApJ...999...59B

We present a large sample of 39 nebular-phase optical spectra of 25 hydrogen-poor superluminous supernovae (SLSNe-I) and jointly analyze them with previously published spectra of 12 events. We measure the properties of key emission features, namely, those at 6300, 7300, and 7774 Å (associated with [O I], [Ca II]/[O II], and O I, respectively), and find that SLSNe exhibit much wider spectral diversity than normal SNe Ic, primarily in the line ratio L₇₃₀₀/L₆₃₀₀, which is highly sensitive to ejecta ionization. Some events exhibit weak [O I] and a clear [O II] contribution to the 7300 Å feature, enhancing the ratio, along with [O III] lines at 4363 and 5007 Å. Other SLSNe show weak or no lines of ionized oxygen. Moreover, we find that the population exhibits decreasing L₇₃₀₀/L₆₃₀₀ over time, while a few outliers instead display sustained high or increasing ratios for extended periods. The ratio L₇₃₀₀/L₆₃₀₀ is also correlated with the rise and decline times of the light curves, with slower events exhibiting higher ionization, the first robust connection between early light-curve and late-time spectral properties, likely due to the magnetar's impact: slower-evolving SLSNe are generally powered by engines with longer spin-down timescales, which deposit more energy at later phases. Among the events with decreasing L₇₃₀₀/L₆₃₀₀, SLSNe with high ionization are on average powered by magnetars with higher thermalized spin-down power, a correlation that is most significant for events with M_ej ≲ 12 M_⊙. The ionization in the outliers with increasing L₇₃₀₀/L₆₃₀₀ may be due to late circumstellar medium interaction. L₇₃₀₀/L₆₃₀₀ and its evolution are therefore key diagnostics of SLSN engines and progenitor mass loss.