Never went away, Linux is now the primary target platform for OpenZFS (which is basically synonymous with ZFS these days). TrueNAS/iXSystems (probably the main commercial company using ZFS) moved from FreeBSD to Linux. Major new features like pool expansion have been added after years of requests. Etc., it's a good time for ZFS on Linux.

There ARE licensing issues related to shipping it compiled into the kernel, but you can install it as a kernel module on every mainline distro nowadays which is functionally the same from a user perspective.