/etc/exim4/conf.d/transport/00_exim4-config_header is in exim4-config 4.90.1-1ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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# TRANSPORTS CONFIGURATION #
######################################################################
# ORDER DOES NOT MATTER #
# Only one appropriate transport is called for each delivery. #
######################################################################
# A transport is used only when referenced from a router that successfully
# handles an address.
begin transports
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