In NOTES file, section "COMPATIBILITY OPTIONS":
# Implement system calls compatible with 4.3BSD and older versions of
# FreeBSD. You probably do NOT want to remove this as much current code
# still relies on the 4.3 emulation. Note that some architectures that
# are supported by FreeBSD do not include support for certain important
# aspects of this compatibility option, namely those related to the
# signal delivery mechanism.
# Old tty interface.
The comment seems outdated since COMPAT_43 has been removed from GENERIC about 10 years now ;)
I'm also suggesting to disable those options by default now on NOTES.
It seems there is another outdated information here:
# Enable 32-bit Linux ABI emulation (requires COMPAT_43 and COMPAT_FREEBSD32)
But according to commit #159651:
"For amd64 there's an explicit comment that it's needed for the linuxolator. This is not the case anymore."