After a apt dist-upgrade I got an email every time a cronjob called apticron:
WARNING: tempfile is deprecated; consider using mktemp instead.
This is the one liner that gives this warning:
if test -x /usr/sbin/apticron; then /usr/sbin/apticron --cron; else true; fi
You can test this by performing this one liner on the command line.
After some digging I found this bugreport (as Kali is a Debian derivative):
This was reported in June 2020. But there is also a patch you can find at the bottom of this bug report. Haven’t tested it, because it’s merely a warning. I expect the maintainer to merge this patch in a next release of apticron.
26-10-2020: After a few of these cronmails I got fed up with that and applied the changes suggested in the patch. Which are really simple, it’s replacing tempfile by mktemp.