OpenZFS is under a completely FOSS license but it’s incompatible with the GPL and can’t really ever be merged into the Linux kernel. The workaroundids to provide it as source code which gets compiled as a module every time there’s a new kernel via dkms.
More controversially, Canonical ship OpenZFS pre-compiled in Ubuntu which some lawyers believe to be infringing on ZFS’ codebase.
Honestly the OpenZFS situation on Linux is probably the biggest single reason for the growing interest in btrfs and bcachefs, the former slowly becoming default on more Linux distros over time and lots of investment from SUSE and Facebook AFAIK.