The Stock Dialer and Contacts apps are officially available on the Store..
Google had previously removed both the apps for unknown reasons, but hey it’s good to have them back. Now the apps can be updated from the Play Store to fix any bugs or add new features directly, instead of waiting for an OTA update to be rolled out in phases.
Oh, and yes, the apps are exclusive to Google Nexus devices. That is probably due to the fact most OEMs ship in their own tweaked or themed version of the Phone and contact apps.