LGA 2066, also called Socket R4, is a CPU socket by Intel that debuted with Skylake-X and Kaby Lake-X processors in June 2017. It replaces Intel's LGA 2011-3 (R3) in the performance, high-end desktop and Workstation platforms (based on the X299 "Basin Falls" and C422 chipsets), while LGA 3647 (Socket P) replaces LGA 2011-3 (R3) in the server platforms based on Skylake-SP (Xeon "Purley").
All of these CPUs require the Intel X299 chipset to work. So, the C422 chipset is strictly limited to work with workstation processors only.
Kaby Lake-X processors were discontinued in May 2018. Starting October 2019, BIOS updates for most of the X299-based motherboards removed support for Kaby Lake-X processors.
Take note that certain types of Xeon processors will not work on this socket, like Skylake-P. There are four OEM variants that can be found in Apple Mac Pro: W-2140B and W-2150B (lower freq/TDP than their W-21x5 counterparts), W-2170B and W-2191B lack Trusted_Execution_Technology (TXT) in comparison.
(* = OEM CPU found in Apple Mac Pro with lower TDP)