Cant’ say it’s that surprising, as last time I did a test with even NetBSD it broke post NetBSD 4.0.1
As they say, with a little testing you can find that stuff has been broken for years without anyone complaining that loud. I guess the lucky thing is that with the rise of fast computers, and decent virtualization and emulation we’ve been running Xenix on Qemu for nearly a decade!
It’s interesting how at one point ABI compatibility was seen as the holy grail of running everything all at once, and micro kernels were going to let us run ‘personalities’ of everything. And we’ve now gone full IBM using hardware virtualization to run paravirtualized guest OS’s at will. Although I still think that OS/2 2.0 had the best paravirtualized guest OS experience, the way it ran Windows seamlessly on the OS/2 desktop.