  Superluminous supernovae (SLSNe) are luminous transients that can be detected
to high redshifts with upcoming optical time-domain surveys such as the Vera C.
Rubin Observatory Legacy Survey of Space and Time (LSST). An interesting open
question is whether the properties of SLSNe evolve through cosmic time. To
address this question, in this paper we model for the first time the
multi-color light curves of all 21 Type I SLSNe from the Dark Energy Survey
(DES) with a magnetar spin-down engine, implemented in the Modular Open Source
Fitter for Transients (MOSFiT). With redshifts up to $z\approx 2$, this sample
includes some of the highest-redshift SLSNe. We find that the DES SLSNe span a
similar range of ejecta and magnetar engine parameters to previous samples of
mostly lower-redshift SLSNe (spin period $P\approx 0.79-13.64$ ms, magnetic
field $B\approx (0.03-7.12)\times10^{14}$ G, ejecta mass $M_{\rm ej}\approx
1.51-30.50$ M$_{\odot}$, and ejecta velocity $v_{\rm ej}\approx
(0.56-1.45)\times 10^4$ km s$^{-1}$). The DES SLSN sample by itself exhibits
the previously found negative correlation between $M_{\rm ej}$ and $P$, with a
pronounced absence of SLSNe with low ejecta mass and rapid spin. Combining our
results for the DES SLSNe with 60 previous SLSNe modeled in the same way, we
find no evidence for redshift evolution in any of the key physical parameters.
