Apple has silently pushed out an update to the XProtect settings for OS X El Capitan and macOS Sierra. These now block the use of Adobe Flash Player earlier than the current version, 23.0.0.205.

If your Flash Player has just stopped working as a result of this, you will need to visit Adobe’s Flash download page, download, and install the current version.

I have noted this update on the listings of these silently updated files for Sierra and El Capitan.