  (Abridged) We present a detailed broadband X-ray spectrum of NGC 4258, with
the goal of precisely measuring the coronal luminosity and accretion flow
properties of the AGN, and track any possible variation across two decades of
observations. We collect archival XMM-Newton, Chandra, Swift/BAT and NuSTAR
spectroscopic observations spanning 15 years, and fit them with a suite of
state of the art models, including a warped disk model which is suspected to
provide the well known obscuration observed in the X-rays. We complement this
information with archival results from the literature. Clear spectral
variability is observed among the different epochs. The obscuring column
density shows possibly periodic fluctuations on a timescale of 10 years, while
the intrinsic luminosity displays a long term decrease of a factor of three in
a time span of 15 years (from $L_{2-10~\text{keV}} \sim 10^{41}$ erg s$^{-1}$
in the early 2000s, to $L_{2-10~\text{keV}} \sim 3 \times 10^{40}$ erg s$^{-1}$
in 2016). The average absorption-corrected X-ray luminosity
$L_{2-10~\text{keV}}$, combined with archival determinations of the bolometric
luminosity, implies a bolometric correction $k_{\rm bol} \sim 20$, intriguingly
typical for Seyferts powered by accretion through geometrically thin,
radiatively efficient disks. Moreover, the X-ray photon index $\Gamma$ is
consistent with the typical value of the broader AGN population. However, the
accretion rate in Eddington units is very low, well within the expected RIAF
regime. Our results suggest that NGC 4258 is a genuinely low-luminosity Seyfert
II, with no strong indications in its X-ray emission for a hot, RIAF-like
accretion flow.
