When running sudo modprobe zram (or modinfo for that matter) I get the following error:
modprobe: FATAL: Module zram not found in directory /lib/modules/4.13.0-21-generic
Has zram been removed from Ubuntu 17.10?
No. Kernel modules are pretty rarely removed from the upstream kernel. However, many modules are split into a separate (versioned) linux-image-extra package in Ubuntu, and you may be missing said package for your kernel version.