The OS X kernel is a mashup of things; it's been too long since I looked, but IIRC the tcp code comes from just before FreeBSD added syncache (released in FreeBSD 4.5), so that should be after the release of jails in 4.0. But, Apple would have needed to integrate jails throughout the NeXT/Mach part of the kernel too.

And, you'd be missing out on features like VNET that came later (FreeBSD 8), unless Apple reintegrated with upstream, which is pretty rare for them. (They did update the FreeBSD userland, once)

I might be entirely off-base with this, but I think it probably has something to do with the roots of macOS' (or really, NeXTSTEP's) BSD component (BSD 4.3) pre-dating FreeBSD 4.0 (the first version that introduced jails) by 14 years.

It does seem like a feature worth porting, though.