To my knowledge, the integration with the kernel is still not very good. As someone else mentioned in a reply, there are large swathes of the Solaris kernel with duplicate functionality included in the Linux kernel solely to support ZFS, because native integration is too difficult in face of the licensing and maintenance issues.
It remains a kludge rather than something that can really be thought of as an integrated Linux solution. Even if it works okay in some situations.
Is the Solaris Porting Layer really due to a licensing issue or just a ZFS portability issue? FreeBSD's ZFS implementation reportedly has a similar portability shim.¹ The new OpenZFS implementations for Windows and macOS each have their own Solaris Porting Layer, too.
If what you want is just snapshotting and you want something that's in the mainline kernel, I guess there's BTRFS. Bcachefs seems to be coming along nicely, but who knows when it'll really be ready.
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1: https://papers.freebsd.org/2019/BSDCan/jude-The_Future_of_Op...
Interestingly, NixOS excels at ZFS integration.
Because NixOS builds everything from source with transparent binary caching, always upgrades your system as an integral while, and performs upgrades atomically, you'll probably not even notice that ZFS isn't in the mainline kernel as you use it. NixOS will never upgrade your kernel to one that's incompatible with whatever version of ZFS is in the repos, and it'll never leave you in a partially upgraded state where your new kernel is installed, but your new ZFS modules are not— even if you pull the power during an upgrade.
ZFS feels very native on NixOS. It's great