Looking closer at Ubuntu releases:

------
Xenial
------

'zfs' meta package is present, and wants to install:

Note, selecting 'zfsutils-linux' instead of 'zfs'
The following package was automatically installed and is no longer required:
  libfreetype6
Use 'sudo apt autoremove' to remove it.
The following additional packages will be installed:
  binutils cpp cpp-5 dkms fakeroot gcc gcc-5 libasan2 libatomic1 libc-dev-bin libc6-dev libcc1-0 libcilkrts5 libfakeroot libgcc-5-dev libgomp1 libisl15 libitm1 liblsan0
  libmpc3 libmpx0 libnvpair1linux libquadmath0 libtsan0 libubsan0 libuutil1linux libzfs2linux libzpool2linux linux-libc-dev make manpages-dev spl spl-dkms zfs-dkms zfs-doc
  zfs-zed
Suggested packages:
  binutils-doc cpp-doc gcc-5-locales gcc-multilib autoconf automake libtool flex bison gdb gcc-doc gcc-5-multilib gcc-5-doc libgcc1-dbg libgomp1-dbg libitm1-dbg
  libatomic1-dbg libasan2-dbg liblsan0-dbg libtsan0-dbg libubsan0-dbg libcilkrts5-dbg libmpx0-dbg libquadmath0-dbg glibc-doc make-doc default-mta | mail-transport-agent
  samba-common-bin nfs-kernel-server zfs-initramfs
The following NEW packages will be installed:
  binutils cpp cpp-5 dkms fakeroot gcc gcc-5 libasan2 libatomic1 libc-dev-bin libc6-dev libcc1-0 libcilkrts5 libfakeroot libgcc-5-dev libgomp1 libisl15 libitm1 liblsan0
  libmpc3 libmpx0 libnvpair1linux libquadmath0 libtsan0 libubsan0 libuutil1linux libzfs2linux libzpool2linux linux-libc-dev make manpages-dev spl spl-dkms zfs-dkms zfs-doc
  zfs-zed zfsutils-linux


------
bionic
------

No 'zfs' meta package present, 'zfsutils-linux' offers to install

The following additional packages will be installed:
  libnvpair1linux libuutil1linux libzfs2linux libzpool2linux zfs-zed
Suggested packages:
  zfs-modules | zfs-dkms nfs-kernel-server samba-common-bin zfs-initramfs | zfs-dracut
The following NEW packages will be installed:
  libnvpair1linux libuutil1linux libzfs2linux libzpool2linux zfs-zed zfsutils-linux


------
cosmic
------

No 'zfs' meta package present, but has a reference to it in the archive.

$ sudo apt install zfs 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package zfs is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'zfs' has no installation candidate

'zfsutils-linux'  offers:

$ sudo apt install zfsutils-linux 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following additional packages will be installed:
  grub-common grub-gfxpayload-lists grub-pc grub-pc-bin grub2-common libnvpair1linux libuutil1linux libzfs2linux libzpool2linux linux-image-unsigned-4.18.0-13-lowlatency
  linux-modules-4.18.0-13-lowlatency os-prober zfs-zed
Suggested packages:
  multiboot-doc grub-emu xorriso desktop-base fdutils linux-doc-4.18.0 | linux-source-4.18.0 linux-tools linux-headers-4.18.0-13-lowlatency nfs-kernel-server
  samba-common-bin zfs-initramfs | zfs-dracut
The following NEW packages will be installed:
  grub-common grub-gfxpayload-lists grub-pc grub-pc-bin grub2-common libnvpair1linux libuutil1linux libzfs2linux libzpool2linux linux-image-unsigned-4.18.0-13-lowlatency
  linux-modules-4.18.0-13-lowlatency os-prober zfs-zed zfsutils-linux


------
disco
------

No 'zfs' meta package but has references in the archive, like cosmic.

$ sudo apt install zfs 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package zfs is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'zfs' has no installation candidate

The 'zfsutils-linux' package offers the following:

$ sudo apt install zfsutils-linux 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following additional packages will be installed:
  grub-common grub-gfxpayload-lists grub-pc grub-pc-bin grub2-common libnvpair1linux libuutil1linux libzfs2linux libzpool2linux linux-image-unsigned-4.18.0-11-lowlatency
  linux-modules-4.18.0-11-lowlatency os-prober zfs-zed
Suggested packages:
  multiboot-doc grub-emu xorriso desktop-base fdutils linux-doc-4.18.0 | linux-source-4.18.0 linux-tools linux-headers-4.18.0-11-lowlatency nfs-kernel-server
  samba-common-bin zfs-initramfs | zfs-dracut
The following NEW packages will be installed:
  grub-common grub-gfxpayload-lists grub-pc grub-pc-bin grub2-common libnvpair1linux libuutil1linux libzfs2linux libzpool2linux linux-image-unsigned-4.18.0-11-lowlatency
  linux-modules-4.18.0-11-lowlatency os-prober zfs-zed zfsutils-linux