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
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
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.