Linus and Greg K-H have both made statements and actions that make it clear that they are philosophically opposed to ZFS as an implementation ("more of a buzzword than anything else"), regardless of licensing issues. They want you to use BTRFS instead, and are going to throw up arbitrary barriers to make ZFS usage and development as painful as possible (like re-licensing non-GPL symbols that they notice ZFS using out from underneath it into GPL).
https://arstechnica.com/gadgets/2020/01/linus-torvalds-zfs-s...
Like, people make a huge racket about how NVIDIA's driver isn't in the kernel and how that means it will never be suitable for day to day use in a desktop environment, because it's out-of-tree and thus will always be in a place of reacting to changes in the kernel as an external team rather than being supported by the kernel team itself. And that's exactly the same situation ZFS is in and will always be in on Linux. And in fact the situation is actually worse than that, while CDDL is open-source it is incompatible with copyleft licenses, by design and by selection when Sun licensed the code. And then you've got the fact that the kernel team is stacked with BTRFS fanboys who are thinking and acting with their loyalties.
I get the desire to improve ZFS on Linux support but the FreeBSD version probably should have remained the repository of record so that ZFS could have been a first-class-citizen in at least one distro. The Linux team is legally and personally opposed to ZFS, it's ZFS choosing to place their focus on improving an abusive relationship.