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 https://github.com/owncloud/client/wiki/OS-Support-Policy#ubuntu-lts 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: https://central.owncloud.org/t/appimage-for-desktop-client-test-it-now/7885. This allows a more flexible/agnostic installation regardless of the system. You can track this in https://github.com/owncloud/client/issues/5951.
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 (https://blog.jessfraz.com/post/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!