Under FAA regs anything on the CE500 rating may be flown single pilot, providing that the pilot has the SP waiver. (Initial check ride in the aircraft, not sim, and renewed annually in the sim).
The 560 Encore is a CE500 type, but the 560 XL and XLS require separate ratings, just as do the C525's CJ1's2's etc.
My SP rating expired more than 10 years ago but I have flown all CE 500's as single pilot, including the Encore. However I have never heard of a single pilot waiver covering the 560 XL.
I wouldn't fly single pilot in any of these types now.