Kali Linux is not based on stable releases of Debian. This means that it is neither based on version 7 or 8 or 9 or whatever. Kali Linux is based on ‘testing’ version of Debian.

Is Kali Linux Debian 9?

The Kali Linux distribution is based on Debian Testing. Therefore, most of the Kali packages are imported, as-is, from the Debian repositories.

What version of Debian is Kali?

In my opinion, it also happens to be one of the best Debian GNU/Linux distributions available. It is based on Debian stable (currently 10/buster), but with a much more current Linux kernel (currently 5.9 in Kali, compared to 4.19 in Debian stable and 5.10 in Debian testing).