The open-source IllumOS code in FreeBSD had fallen behind Linux ZoL (ZFS-On-Linux). FreeBSD adopted ZoL to keep up with new bugfixes, enhancements, etc.
>>The OpenZFS project brings together developers from the Linux, FreeBSD, illumos, MacOS, and Windows platforms. OpenZFS is supported by a wide range of companies.
https://www.freebsdnews.com/2019/01/10/zfs-on-freebsd-zof-is...
So, for open-source, ZoL is the 'mainstream' ZFS.