I believe so. The package descriptions for most of the ZFS packages in Ubuntu mention OpenZFS, so it certainly appears that way.
You can still create pools that are compatible with Oracle Solaris, you just have to set the pool version to 28 or older when you create it and obviously don’t update it. That will prevent you from using any of the newer features that have been added since the fork.
I believe so. The package descriptions for most of the ZFS packages in Ubuntu mention OpenZFS, so it certainly appears that way.
You can still create pools that are compatible with Oracle Solaris, you just have to set the pool version to 28 or older when you create it and obviously don’t update it. That will prevent you from using any of the newer features that have been added since the fork.