  Ground-based optical spectra and Hubble Space Telescope images of ten
core-collapse supernovae (CCSNe) obtained several years to decades after
outburst are analyzed with the aim of understanding the general properties of
their late-time emissions. New observations of SN 1957D, 1970G, 1980K, and
1993J are included as part of the study. Blueshifted line emissions in oxygen
and/or hydrogen with conspicuous line substructure are a common and
long-lasting phenomenon in the late-time spectra. Followed through multiple
epochs, changes in the relative strengths and velocity widths of the emission
lines are consistent with expectations for emissions produced by interaction
between SN ejecta and the progenitor star's circumstellar material. The most
distinct trend is an increase in the strength of [O III]/([O I]+[O II]) with
age, and a decline in Halpha/([O I]+[O II]) which is broadly consistent with
the view that the reverse shock has passed through the H envelope of the ejecta
in many of these objects. We also present a spatially integrated spectrum of
the young Galactic supernova remnant Cassiopeia A (Cas A). Similarities
observed between the emission line profiles of the 330 yr old Cas A remnant and
decades old CCSNe suggest that observed emission line asymmetry in evolved CCSN
spectra may be associated with dust in the ejecta, and that minor peak
substructure typically interpreted as 'clumps' or 'blobs' of ejecta may instead
be linked with large-scale rings of SN debris.
