The Platform repo (https://github.com/joomla/joomla-platform
) is retired and not receiving updates. We should probably archive it or remove it or make it private or something.
If you want to start a non-CMS project, the Framework is a good place to look. The next major version of the CMS will see integrations with a lot of PHP libraries, both from the Joomla Framework as well as 3rd party libraries like possibly Symfony, Zend, Aura and other 1 off libraries like Flysystem (for filesystem abstraction). The plan is for it also to introduce it's own abstraction layer between these 3rd party libs and the CMS itself so that the underpinnings can be swapped out if needed.
All of this is still up in the air though. No one knows for SURE what CMS v4 will bring.