This is xnu-8019. See this file in:
module Darwin.netinet6 [system] {
  #ifdef XNU_KERNEL_PRIVATE
  // The PLATFORM_MacOSX headers are in the public SDK for macOS,
  // but mastered out of the public SDK for the other OSes.
  #endif
  #ifdef PLATFORM_MacOSX
  module ah {
    header "netinet6/ah.h"
    export *
  }

  module esp {
    header "netinet6/esp.h"
    export *
  }

  module in6_var {
    header "netinet6/in6_var.h"
    export *
  }

  module ipcomp {
    header "netinet6/ipcomp.h"
    export *
  }

  #endif
  module ipsec {
    header "netinet6/ipsec.h"
    export *
  }

  #ifdef PLATFORM_MacOSX
  module raw_ip6 {
    header "netinet6/raw_ip6.h"
    export *
  }

  #endif
  module scope6_var {
    header "netinet6/scope6_var.h"
    export *
  }
  #ifdef XNU_KERNEL_PRIVATE
  // nd6.h doesn't compile because it uses struct prf_ra which
  // only exists in the context of struct in6_prflags.
  #endif
  #ifdef PLATFORM_MacOSX

  exclude header "netinet6/nd6.h"
  #endif
}