It’s part of the MSVC x64 ABI, not a part of the kernel ABI. The kernel and user mode (Win32) APIs all use C linkage so vtables are not relevant and the ABI is a convention of the compiler not the OS. Practically speaking though if your software runs on Windows it will probably use the MSVC ABI.

Last time I checked, software compiled with both Cygwin and MSYS2 internally uses SysV calling convention, only switching to WINAPI when calling, well, Win32 API.