  We present three non-parametric Bayesian primordial reconstructions using
Planck 2018 polarization data: linear spline primordial power spectrum
reconstructions, cubic spline inflationary potential reconstructions and
sharp-featured primordial power spectrum reconstructions. All three methods
conditionally show hints of an oscillatory feature in the primordial power
spectrum in the multipole range $\ell\sim20$ to $\ell\sim50$, which is to some
extent preserved upon marginalization. We find no evidence for deviations from
a pure power law across a broad observable window
($50\lesssim\ell\lesssim2000$), but find that parameterizations are preferred
which are able to account for lack of resolution at large angular scales due to
cosmic variance, and at small angular scales due to Planck instrument noise.
Furthermore, the late-time cosmological parameters are unperturbed by these
extensions to the primordial power spectrum. This work is intended to provide a
background and give more details of the Bayesian primordial reconstruction work
found in the Planck 2018 papers.
