It's a duplicate of bug 3499558, but I'm not alowed to reopen it.
As per doxygen's changelog [1]:
The tool doxytag has been declared obsolete and is removed (it wasn't working properly anyway). Same goes for the installdox script.
Downstream bug [2]
[1] http://www.stack.nl/~dimitri/doxygen/changelog.html
[2] https://bugs.gentoo.org/show_bug.cgi?id=406849