From the article, the time to beat is November 16, 1992. This might be possible on one of the BSDs, as FreeBSD and NetBSD both document support for running a.out executables with the 4.3BSD ABI. I guess it would depend on whether anyone had ported X11 to run on a BSD on i386 before then. Alternatively, maybe you'd be able to get something running on SCO OpenServer (if you count that as a current OS, it IS still getting security patches). It documents support for XENIX x.out binaries ( https://www.scosales.com/ta/kb/126704.html ) so you might be able to run software compiled in the late 1980s for XENIX/386.
replyNetBSD has excellent binary compatible with 80s "unix", the problem is most (all?) of the 80s unix platforms it supports aren't x86.
replyCurrent NetBSD still has good support for running SunOS binaries, though, including graphical ones. I'm certain you could get a SunOS 4.x binary from pre-1992 to run on NetBSD/SPARC. I've no idea how far back you could potentially go on NetBSD/m68k -- probably quite a ways, especially if the binaries are statically linked. SunOS pre-1988 was all static linking!
(Running "real" 4.3BSD binaries might be hard -- I think NetBSD/VAX defaults to ULTRIX compat, which is an either/or thing versus strict Berkeley Unix compatibility. Same binary format, similar but incompatible syscalls/data structures.)