Due to security issues in older versions, Apple has updated the
web plug-in blocking mechanism to disable all versions prior to
Flash Player 126.96.36.1995 and 188.8.131.529.
Time to update your flash player once again. If you have windows, you should do it too, because Microsoft is content to let you run out of date vulnerable versions for years.
Just got a question about metadata and whether it is removed when you flatten a PDF.
Removing metadata is a more thorough process than just flattening a PDF
see the following link:
So the quick answer is no, flattening does not insure that the metadata is removed.