Extended reionization in models beyond $\Lambda$CDM with Planck 2018
  data
  We provide an update on the constraints on extended reionization histories
with the Planck 2018 cosmic microwave background anisotropy data. The Planck
2018 data on large angular scales improve the measurement of the $E$-mode
polarization reionization bump at low multipoles providing the possibility to
improve our previous results. Using a minor modification to the original
Poly-reion model for the reionization history, we find that the Planck 2018
data significantly improve all our previous results: we find as optical depth
of $\tau=0.0572_{-0.0075}^{+0.0064}$ at 68% CL, that early onsets of
reionization are strongly disfavoured, i.e. redshift when the reionization
begins, $z_{xe=0}=18.18_{-10.89}^{+1.61}$ at 68% CL,and that reionization
duration (defined between 10% and 99% reionization) is significantly reduced,
i.e. $\Delta_z^{Reion}=4.59_{-2.45}^{+1.67}$ at 68% CL. We explore possible
correlations between reionization histories and cosmological parameters,
including important extensions beyond $\Lambda$CDM. We find that the degeneracy
between reionization and scalar spectral index,neutrino mass sum, spatial
curvature, dark matter annihilation and other non-standard models are
significantly reduced.The reduction of the error bars and the degeneracies,
together with the shift towards lower values of the optical depth that we
observe in the Poly-reion model are mainly driven by the new low-$\ell$
polarization likelihood of Planck 2018 baseline based on the HFI data. This is
confirmed also by the results derived without this likelihood and the ones with
different alternatives to the baseline that are presented for a subset of
models.
