Is the reason this (or something substantially similar) is missing from OpenBSD that we don't want it, or just that nobody has done the work to implement it?
If it is the latter, I can try to make a patch.
{Free,Net}BSD have wait6(2), which is even more general than wait4(2),
and allows one to implement POSIX-mandated waitid(2). See e.g.
https://man.netbsd.org/wait.2 .
