The bsd binary compatibility system was an interesting effort to allow executeable files built for other systems to run on bsd, I never actually used it but my understanding is it was always hard to set up and fragile.
OpenBSD removed it as they did not feel they could properly maintain the system.
FreeBSD keeps a little of it around, mainly to run linux binaries.
... but the BSDs did have support for SystemV binaries, through modules compat_svr4 and compat_ibcs2, so the grand parent comment made perfect sense to me. Not sure why there were two and I don't plan to go down that rabbit hole today, but perhaps due to SVR4 vs SVR3 differences, and ELF vs COFF, shrug.
OpenBSD removed it as they did not feel they could properly maintain the system.
FreeBSD keeps a little of it around, mainly to run linux binaries.
NetBSD it looks like they kept the whole buffet.
https://www.netbsd.org/docs/compat.html