/etc/exim4/conf.d/router/00_exim4-config_header is in exim4-config 4.76-3ubuntu3.4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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# ROUTERS CONFIGURATION #
# Specifies how addresses are handled #
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# THE ORDER IN WHICH THE ROUTERS ARE DEFINED IS IMPORTANT! #
# An address is passed to each router in turn until it is accepted. #
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begin routers
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