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.ยน 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.
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.