> Since then, illumos has rewritten all those components
But apparently kept the same license?
>> Most of the existing code is licensed under the CDDL and we expect new code will generally be under this license as well.[0]
Whether you use it or not the Illumos code base[1] is one of the most well written I've seen in this respect.
The various Illumos distributions routinely upstream their changes into the master Illumos repository ( https://github.com/illumos/illumos-gate). I know that ZFS on Linux (https://github.com/zfsonlinux/zfs) routinely integrates those changes, as well as making some of its own and I believe pushing them upstream to Illumos. When I've looked at FreeBSD commit logs, they too are pulling in ZFS changes from Illumos on a frequent basis (but I know less about any independent development and upstreaming).
My strong impression is that the ZFS code base is a sufficiently big and tangled thing that no one wants it to fragment. With bug-fixes and improvements happening in Illumos, ZoL and FreeBSD both want to be able to incorporate them on a regular basis and to push their own changes upstream to reduce the maintenance burden of carrying those changes.
It's not Solaris -- it's illumos and has been for a long time. And the source code is available on GitHub.[1]
For the record, there is no "proprietary [operating system]" involved here. The Triton stack (upon which Manta is based) runs on top of SmartOS[1], a wholly open source operating system built around an illumos[2] core.