/usr/share/doc/util-linux/examples/fstrim.service is in util-linux 2.29.2-1+deb9u1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | [Unit]
Description=Discard unused blocks
[Service]
Type=oneshot
ExecStart=/sbin/fstrim -av
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