Hi. Over at
https://www.boost.org/doc/libs/1_72_0/libs/filesystem/doc/reference.html#path-extension  
(and older version of the doc too) there's a link to
http://permalink.gmane.org/gmane.comp.lib.boost.devel/199744
which is no longer so permanent :). According to https://en.wikipedia.org/wiki/Gmane
the website is no longer available. I guess any GMane link in docs should be updated
to another ML archive. My $0.02. Thanks, --DD