/usr/share/texlive/texmf-dist/tex/xmltex/base/utf-16.xmt is in texlive-htmlxml 2015.20160320-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | % not a serious implementation
\input iso-8859-1.xmt
\catcode0=9
|