Question about netinet6/in6.h

This has to be a FAQ

I'm porting a program from Linux, I often see an error like:
./test/mock-ifaddrs.c:95:19: error: no member named 's6_addr32' in 'struct
in6_addr'
   95 |                 ipv6->sin6_addr.s6_addr32[3] = 0;
      |                 ~~~~~~~~~~~~~~~ ^
but yet, we kinda define them, but only for the kernel and boot loader:
/*
 * IPv6 address
 */
struct in6_addr {
        union {
                uint8_t         __u6_addr8[16];
                uint16_t        __u6_addr16[8];
                uint32_t        __u6_addr32[4];
        } __u6_addr;                    /* 128-bit IP6 address */
};

#define s6_addr   __u6_addr.__u6_addr8
#if defined(_KERNEL) || defined(_STANDALONE) /* XXX nonstandard */
#define s6_addr8  __u6_addr.__u6_addr8
#define s6_addr16 __u6_addr.__u6_addr16
#define s6_addr32 __u6_addr.__u6_addr32
#endif

I'm wondering if anybody why it's like that? git blame suggests we imported
that from kame, with
only tweaks by people that are now deceased*.*

Why not just expose them?

Warner