/usr/bin/unihexfill is in unifont-bin 1:10.0.07-1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | #!/bin/sh
# input is a start, stop range pair
# output is a list of code points with hexadecimal filler glyphs
set -e
grep "^[0-9A-Fa-f]" $1 | \
awk 'NF == 1 {print "unihexgen " $1 " " $1;} \
NF == 2 {print "unihexgen " $1 " " $2;}' | \
/bin/sh -s
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