ZFS also had expansion for a long time but it was offline expansion. I don't know if btrfs has also had online for a long time?

And shrinking no, that is a big missing feature in ZFS IMO. Understandable considering its heritage (large scale datacenters) but nevertheless an issue for home use.

But raidz is rock-solid. Btrfs' raid is not.

Raidz wasn't able to be expanded in place before this. You were able to add to a pool that included a raidz vdev, but that raidz vdev was immutable.

Oh ok, I've never done this, but I thought it was already there. Maybe this was the original ZFS from Sun? But maybe I just remember it incorrectly, sorry.

I've used it on multi-drive arrays but I never had the need for expansion.

You could add top level raidz vdevs or replace the members of a raid-z vdev with larger disks to increase storage space back then. You still have those options now.

You can online expand a ZFS pool for years by replacing disks with larger ones or by adding vdevs. On current OpenZFS you can even expand a Raid-Z by adding disks to it.

To shrink a pool, you can remove vdevs. On OpenZFS this is limited to mirror vdevs, native ZFS from Oracle can remove Raid-Z.

Kernel integration is not so important as long as use distributions that include ZFS support at distribution level mainly Proxmox and TrueNAS or Ubuntu.