Symfony 3.3 adds a new versioning strategy based on JSON file manifests, which is used by popular tools such as Webpack.
Symfony 3.3 deprecates the ConsoleEvents::EXCEPTION event in favor of a new ConsoleEvents::ERROR event that is able to handle also the exceptions thrown from listeners.
Symfony 3.3 adds a new Lock component that supports different stores (flock, semaphore, Redis, Memcache), auto-expiring locks and combining multiple lock stores.
Symfony 3.3 adds full support for loading config files using glob patterns, both for the container and the routing file loaders.
Symfony 3.3 deprecates the warmup part of the cache:clear because it does not work well and it doesn't guarantee that the generated cache is exactly the same as the one that would have been generated via cache:warmup.
Symfony 3.3 adds new features to simplify services configuration thanks to the default config options and the interface-based config options.
In Symfony 3.3 it will be easier to write applications that don't use bundles thanks to the new build() method added to the Kernel class.
Symfony 3.3 has deprecated the X-Status-Code HTTP header used to set custom status codes when dealing with exceptions.
Symfony 3.3 has deprecated the entire ClassLoader component because, in the new PHP 7 era, it doesn't provide any significant performance improvement.
Symfony 3.3 added support for preloading web assets with HTTP/2 Push, a technique that allows web servers to send resources to a web browser before the browser gets to request them.