  Over the last four decades, persistent and flaring emission of magnetars
observed by various telescopes has provided us with a suite of light curves and
spectra in soft and hard X-rays, with no emission yet detected above around 1
MeV. Attenuation of such high-energy photons by magnetic pair creation and
photon splitting is expected to be active in the magnetospheres of magnetars,
possibly accounting for the paucity of gamma-rays in their signals. This paper
explores polarization-dependent opacities for these two QED processes in static
vacuum dipole magnetospheres of highly-magnetized neutron stars, calculating
attenuation lengths and determining escape energies, which are the maximum
photon energies for transparency out to infinity. The numerical trajectory
integral analysis in flat and curved spacetimes provides upper bounds of a few
MeV or less to the visible energies for magnetars for locales proximate to the
stellar surface. Photon splitting opacity alone puts constraints on the
possible emission locales in their magnetospheres: regions within field loops
of maximum altitudes 2-4 stellar radii are not commensurate with maximum
detected energies of around 250 keV. These constraints apply not only to
magnetar flares but also to their quiescent hard X-ray tail emission. An
exploration of photon splitting attenuation in the context of a resonant
inverse Compton scattering model for the hard X-ray tails derives distinctive
phase-resolved spectroscopic and polarimetric signatures, of significant
interest for future MeV-band missions such as AMEGO and e-ASTROGAM.
