There’s a thing called OpenZFS. With ZFS happened almost the same thing as with Java. Oracle bought a company and tried to close ZFS, but people just reimplemented ZFS under a FOSS licence and community. I don’t know who uses Oracle ZFS nowadays. Everyone uses OpenZFS.

It’s true that there’s some licence incompatibility that doesn’t allow integrate OpenZFS into a Linux core, but it’s not like ZFS is proprietary

https://openzfs.github.io/openzfs-docs/License.html

While both (OpenZFS and Linux Kernel) are free open source licenses they are restrictive licenses. The combination of them causes problems

Why btrfs and not ZFS? In my info bubble, the btrfs has a reputation of an unstable FS and people ended up with unrecoverable data.