Comparison of the spectra of SN 2018ibb to those of other slow-evolving SLSNe between t_max+30 days and t_max+1000 days (darker colour: 5 Å binning; light shade: unbinned spectra). Photospheric phase (Panels A–B): Around t_max (Panel A), the spectra of PTF12dam and SN 2015bn are characterised by a hot continuum with superimposed O II absorption lines as seen in many SLSNe at a similar epoch. SN 2007bi, LSQ14an and SN 2018ibb have cooler photospheres, and their spectra exhibit absorption lines from Ca, Fe, Mg, O and Si (see Fig. 7 for their locations) but not O II. At around t_max+60 days (Panel B), all spectra appear similar, though differences exist. LSQ14an, PS1-14bj, and SN 2018ibb are the only SLSNe showing [O III] in emission. Furthermore, SN 2007bi, LSQ14an and SN 2018ibb exhibit strong [Ca II]+[O II] in emission. This feature is also present in PTF12dam and SN 2015bn but is less pronounced. Nebular phase (Panels C–E): Differences start to emerge during the early nebular phase and become stronger with time. SN 2018ibb, LSQ14an and PS1-14bj continue to show conspicuous [O III] in emission, in contrast to PTF12dam, SN 2007bi and SN 2015bn that have very strong [O I] and O I in emission. SNe 2015bn and 2018ibb are the only SLSNe with spectra at ∼t_max+1000 days (Panel E). SN 2018ibb continues to show intermediate-width [O III], whereas the spectrum of SN 2015bn exhibits [O I]. The elevated noise in the SN 2018ibb spectrum at t_max+989.2 days at λ > 6000 Å is due to residuals of the skyline subtraction. The dashed vertical lines indicate the expected locations of emission lines commonly seen from H II regions.