If si_code is equal to CLD_EXITED, then si_status holds 
the least significant eight bits of the exit value of 
the process (with the remaining bits unspecified);
otherwise, it is equal to the signal that caused the 
process to change state.

I can't think of a reason that 32-bit exit statuses would be problematic for the kernel offhand.  Th
ere's not internal kernel ABI to worry about in Linux.  RHEL might have a problem, but life is hard 
for RHEL.

The libc question is... trickier.  I think the realization of this change in glibc would turn on the
 precise language around si_status. The POSIX manual page (my copy of 'man 3p waitid' anyway) doesn'
t mention si_status at all.  The Linux manual page says:

 si_status   Either  the  exit status of the child, as given to _exit(2)
             (or exit(3)), or the signal that caused the child to termi‐
             nate,  stop, or continue.  The si_code field can be used to
             determine how to interpret this field.

On that basis I think the text in:

http://austingroupbugs.net/view.php?id=594#c1317 [^]

is plausible, since it basically says the same thing, look at si_code first.  There might be binary-
compatibility reasons to want to version the libc symbols for waitid() and sigaction() so that old a
pps get precisely the same bits they always did while new apps are expected to get it right, but tha
t's a glibc maintainer decision.