/etc/texmf/texmf.d/20xmltex.cnf is in xmltex 1.9.debian.1-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | % Settings for Debian xmltex
TEXINPUTS.pdfxmltex = .;$TEXMF/tex/{xmltex,latex,plain,generic,}//
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