It's had the ``Not implemented'' notice since ksh.1's initial import 19 years ago [1], so it's probably not going to be implemented.
[1]: http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/bin/ksh/ksh.1?rev=1.1&content-type=text/x-cvsweb-markup Index: ksh.1 =================================================================== RCS file: /cvs/src/bin/ksh/ksh.1,v retrieving revision 1.171 diff -u -p -r1.171 ksh.1 --- ksh.1 24 Nov 2015 21:07:31 -0000 1.171 +++ ksh.1 25 Dec 2015 18:32:46 -0000 @@ -1386,12 +1386,6 @@ is set, it overrides If this parameter is found to be set after any profile files are executed, the expanded value is used as a shell startup file. It typically contains function and alias definitions. -.It Ev ERRNO -Integer value of the shell's -.Va errno -variable. -It indicates the reason the last system call failed. -Not yet implemented. .It Ev EXECSHELL If set, this parameter is assumed to contain the shell that is to be used to execute commands that
