/usr/share/mate-panel/ is in ubuntu-mate-default-settings 18.04.17.
This file is owned by root:root, with mode 0o755.
..
/usr/share/mate-panel/layouts/
/usr/share/mate-panel/layouts/contemporary.layout
/usr/share/mate-panel/layouts/eleven.dock
/usr/share/mate-panel/layouts/eleven.layout
/usr/share/mate-panel/layouts/familiar.layout
/usr/share/mate-panel/layouts/mutiny.layout
/usr/share/mate-panel/layouts/mutiny.panel
/usr/share/mate-panel/layouts/netbook.layout
/usr/share/mate-panel/layouts/netbook.panel
/usr/share/mate-panel/layouts/pantheon.dock
/usr/share/mate-panel/layouts/pantheon.layout
/usr/share/mate-panel/layouts/redmond.layout
/usr/share/mate-panel/layouts/ubuntu-mate.layout