  High-temperature superconductivity is reported in a series of
compositionally-complex cuprates with varying degrees of size and spin
disorder. Three compositions of Y-site alloyed YBa$_2$Cu$_3$O$_{7-x}$, i.e.,
(5Y)BCO, were prepared using solid-state methods with different sets of rare
earth ions on the Y-site. Synchrotron X-ray diffraction and energy-dispersive
X-ray spectroscopy confirm these samples have high phase-purity and homogeneous
mixing of the Y-site elements. The superconducting phase transition was probed
using electrical resistivity and AC magnetometry measurements, which reveal the
transition temperature, T$_C$, is greater than 91 K for all series when near
optimal oxygen doping. Importantly, these T$_C$ values are only $\approx$1$\%$
suppressed relative to pure YBCO (T$_C$ = 93 K). This result highlights the
robustness of pairing in the YBCO structure to specific types of disorder. In
addition, the chemical flexibility of compositionally-complex cuprates allows
spin and lattice disorder to be decoupled to a degree not previously possible
in high-temperature superconductors. This feature makes compositionally-complex
cuprates a uniquely well-suited materials platform for studying proposed
pairing interactions in cuprates.
