Hey @J5fine! Thanks for your interest on the client! Indeed no packages were built for Ubuntu versions older than 14.04 since that requires maintaining lots of different configurations on our build system. We have an OS Support Policy OS Support Policy · owncloud/client Wiki · GitHub where we explain this:
Ubuntu LTS
ownCloud Client will be provided and supported the latest and second to latest version of any given version.
In a future release we’ll also start to provide (at the same time as the packages) an Appimage like @dragotin described in: AppImage for Desktop Client - test it now!. This allows a more flexible/agnostic installation regardless of the system. You can track this in [Linux] Also supply AppImage? · Issue #5951 · owncloud/client · GitHub.
I also gave a small talk on universal Linux application distribution on this year’s #oCConf (https://youtu.be/nxQkM6nxW2s?t=15m49s) on how you can run any GUI desktop application inside a Docker container (Ramblings from Jessie: Docker Containers on the Desktop) by mounting the X11 socket on your host system. You can write down a small Dockerfile based on a supported distribution where you install the client an run it on top of your system if you want to try it out.
Hope this helps!