  Following our recent work on Type II supernovae (SNe), we present a set of 1D
nonlocal thermodynamic equilibrium radiative transfer calculations for
nebular-phase Type Ibc SNe starting from state-of-the-art explosion models with
detailed nucleosynthesis. Our grid of progenitor models is derived from He
stars that were subsequently evolved under the influence of wind mass loss.
These He stars, which most likely form through binary mass exchange, synthesize
less oxygen than their single-star counterparts with the same zero-age main
sequence (ZAMS) mass. This reduction is greater in He-star models evolved with
an enhanced mass loss rate. We obtain a wide range of spectral properties at
200d. In models from He stars with an initial mass >6Msun, the [OI] 6300, 6364
is of comparable or greater strength than [CaII] 7291,7323 -- the strength of
[OI] 6300, 6364 increases with He-star initial mass. In contrast, models from
lower mass He stars exhibit a weak [OI] 6300, 6364, strong [CaII] 7291,7323,
but also strong NII lines and FeII emission below 5500A. The ejecta density,
modulated by the ejecta mass, the explosion energy, and clumping, has a
critical impact on the gas ionization, line cooling, and the spectral
properties. FeII dominates the emission below 5500A and is stronger at earlier
nebular epochs. It ebbs as the SN ages, while the fractional flux in [OI] 6300,
6364 and [CaII] 7291,7323 increases, with a similar rate, as the ejecta
recombine. Although the results depend on the adopted wind mass loss rate and
pre-SN mass, we find that He stars of 6-8Msun initially (ZAMS mass of
23-28Msun) match adequately the properties of standard SNe Ibc. Our results for
less massive He stars are more perplexing, since the predicted spectra are not
seen in nature. They may be missed by current surveys or associated with Type
Ibn SNe in which interaction dominates over decay power. [Abridged]
