I figured it'd be more like how proton provides windows APIs on Linux and applications "just work" as per normal.
I admire your purist approach, but most folks don't have that luxury and just need to make do with what works today for their tooling of choice (or more common, what their employer thrusts upon them.)
BSD is more for purists anyway. Virtualization seems to be a better option than compatibility layers for the odd program that doesn't work natively.
Maybe that it's different for Windows API's on Linux, because by virtualizing Windows, you're still dealing with an unfree OS.
(Since then, OpenBSD has gained support for virtualization for some operating systems including Linux, through the vmm(4) hypervisor.)
> OpenBSD: Whatever the C compiler produced last is the only OpenBSD ABI :D
OpenBSD, well, by policy running old, insecure binaries it's basically a no-no.
If the bsd posix equivalent is the highest layer your willing to use, you'll miss out on some great software.