/lib/partman/choose_partition/80undo/choices is in ubiquity 2.10.16.
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 | #!/bin/sh
. /usr/share/debconf/confmodule
db_metaget partman/text/undo_everything description
printf "undo\t$RET"
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