The generated shell script (-o:sh) uses bash features, for example flags of the echo command which are undefined in POSIX shells, so it should use the correct shebang line:

#!/bin/bash

Without this, running the script on systems where /bin/sh is not bash results in garbled output (for example on macOS):

-n
[   0%] -n
 Keeping:   /tmp/test/a.jpg
-n
[  50%] -n
 Deleting:  /tmp/test/b.jpg
-n
[ 100%] -n
 Done!
Deleting script  /tmp/rmlint.sh