I'm not part of zfs on linux, I just looked at their github milestones[1]. It looks like they're reserving the 1.0 version for when it's "Fully functional and feature complete native ZFS implementation for Linux." Right now it's just usable and stable. Their previous milestones are also interesting. v0.6.0 had several RCs before being released as stable.[2]

[1]: https://github.com/zfsonlinux/zfs/issues/milestones

[2]: https://github.com/zfsonlinux/zfs/issues/milestones?state=cl...