  The light curve diversity of hydrogen-poor superluminous supernovae (SLSNe)
has kept open the possibility that multiple power sources account for the
population. Specifically, pair-instability explosions (PISNe), which produce
large masses of $^{56}$Ni, have been argued as the origin of some
slowly-evolving SLSNe. Here we present detailed observations of SN 2016inl
(=PS16fgt), a slowly-evolving SLSN at $z=0.3057$, whose unusually red spectrum
matches PS1-14bj, a SLSN with an exceptionally long rise time consistent with a
PISN. Ground-based and Hubble Space Telescope data, spanning about 800
rest-frame days, reveal a significant light curve flattening, similar to that
seen in SN 2015bn, and much slower than the decline rate expected from
radioactive decay of $^{56}$Co. We therefore conclude that despite its slow
evolution, SN 2016inl is inconsistent with a PISN. Instead, the light curve
evolution matches the expected power-law spin-down of a magnetar central
engine, but with a shallower power law ($L\propto t^{-2.8}$) compared to that
in SN 2015bn, indicating a possible difference in the $\gamma$-ray opacity
between the two events. Analytical modeling indicates typical magnetar engine
parameters, but one of the highest ejecta masses ($\approx 20$ M$_{\odot}$)
inferred for a SLSN. Our results indicate that monitoring the late-time light
curve evolution of SLSNe provides a powerful diagnostic of their energy source.
