[yjftsjthsd-h]

My impression is that Darwin did it by moving more drivers directly into user space. But yes, you can absolutely run Linux with everything statically compiled into the kernel as long as you're not using some handful of things that resist it (below comment mentions nvidia, ZFS). You can even run without an initial ramdisk if you're not doing RAID or ZFS or encrypted disks or something like that.

Edit: I should mention, this will either result in a massive kernel that consumes a lot of memory, or in very little driver support and your machine will not tend to just work when you plug new devices in. Linux has a lot of drivers; there's a reason why it uses modules.

[bombcar]

I wonder if you can force the code to compile ZFS in, since the license problem is one of distribution not of user/runtime.

Ubuntu might not be able to distribute said "no module" kernel, but it might run.

[yjftsjthsd-h]

I believe ZFS at least used to have an option to insert itself directly into a Linux source tree, in which case it would look just like a normal driver. I don't know if that still exists and I never tried it, but it was a thing. Note that you probably still need an initial ramdisk to get the userspace tools to actually bring a pool online if you're using it for root.

Edit: I'm having trouble finding it in the official documentation, but here's a page that describes how to do it on an old version: https://slackwiki.com/ZFS_root_(builtin) and here's what looks like a script to do that on the current tip of master: https://github.com/openzfs/zfs/blob/master/copy-builtin