Apple today published a support document explaining that users with out-of-date versions of the Adobe Flash Player plug-ins will see a "Blocked plug-in," "Flash Security Alert," or "Flash out-of-date" message when attempting to view Flash content in Safari.
Plug-ins like Adobe Flash Player have long been an issue for Apple, requiring forced updates and security fixes to patch vulnerabilities. When vulnerabilities arise, Apple has been consistent in blocking older versions of the web plug-ins. Apple is looking to reduce the risk of potential issues with macOS Sierra, in which Safari will deactivate Flash Player and other plug-ins by default in an effort to push the more modern HTML5.
To continue using Flash, users must download the latest Adobe Flash Player update from Adobe's website.