/etc/Muttrc.d/sidebar.rc is in mutt-patched 1.5.24-1build1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | # Configuration for the sidebar patch.
# See /usr/share/doc/mutt/README.Patches for documentation.
ifdef sidebar_visible set sidebar_visible sidebar_width=20
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