adobe-flash package is inventing a local npapi USE flag instead of using the global nsplugin USE flag. Please consider using the widely used IUSE=nsplugin instead. Default enabling it still sounds good. Also note that both npapi and ppapi local USE flag descriptions have an opening parenthesis, but no closing one.
<flag name="nsplugin">Install the Netscape Flash API plugin (for Firefox and similar browsers</flag> <flag name="ppapi">Install the Pepper Flash API plugin (for Chrome and similar browsers</flag> Closing parenthesis still missing