Liferay CAS client was deprecated from version 7.3 and removed completely in 7.4. This repository demonstrates how to bring it back to Liferay.
If you just want to have the CAS working in your portal, download the three attached binaries and put them to your LIFERAY_HOME/deploy folder (each release contains Liferay GA and cas-client-core versions).
You can find more information about this repository on a blog post I published on the Liferay community page.
The file GETTING_STARTED.markdown comes with Liferay Workspace by default, and provides some documentation about it (I'm not the author).