Linux users are constantly using drivers outside of the mainline kernel. Whether it’s graphics cards, radio drivers (things have gotten better in that regard but Bluetooth is support is still terrible) or FUSE file systems.

The difference with ZFS is that the code is kernel-ready but there’s just some licensing worries (understandable ones) that stop it from being mainlined.

I’ve been running ZFS on Ubuntu Server for several years now and frankly ZFS is the only part of that entire system that doesn’t suck (in my opinion). I’d switch back to FreeBSD in a heartbeat if I didn’t need Docker support but credit where credit is due, Ubuntu’s ZFS support has been really good.

Edit: just to add, I’ve got nothing against anyone who does enjoy Ubuntu Server. It’s just not a Linux distro I personally have much fondness for.