The biggest addition is support for PSR-6. Stash now implements the various interfaces natively, allowing it to be directly injected into PSR-6 compliant libraries.
This did require a few API changes (which prompted some cleanup of deprecated functions), so please make sure to review the release notes.
Stash remains one of the best tested caching libraries out there, and this improved on that significantly. In addition there have been optimizations and improvements in PHP7 support, including behind the scenes switching between the APC and APCu functions.
The Symfony Stash Bundle has also been updated, with version v0.6.1 utilizing the Stash v0.14 line.