Kali and Debian
Kali has detailed this in their blog post "Kali's Relationship With Debian".
The Kali Linux distribution is based on Debian Testing. Therefore, most of the Kali packages are imported, as-is, from the Debian repositories. In some cases, newer packages may be imported from Debian Unstable or Debian Experimental, either to improve user experience, or to incorporate needed bug fixes.
Debian Testing is a rolling development release.
Debian testing is the current development state of the next stable Debian distribution. It is also made available under the code name of the next stable release, Bullseye is the current testing codename.
So in short, they're not based on A Debian Release. Like Ubuntu and many others, Kali does source their packages from Debian.
Don't create a FrankenDebian
What you're wanting to do is what we call making a "FrankenDebian". Because Debian Testing is rolling any version of it will be different from any released or frozen version of Debian. So the question amounts to more or less,
At the time I'm doing this what release of Debian is closest to the release that Kali took when they froze Debian Testing.
That's going to be very contemporary,
- if Kali sourced testing after Debian froze testing and made a release then Kali may have a newer version than any released version of Debian.
- if Kali sourced testing before Debian froze testing and made a release then they're going to likely have an older version of said things.
This is will not be good for you either way.
A way out with Containers
A better idea is to simply use Debian Stable (which Oracle supports) and run Kali in a container. You can find information on the Kali site about using their Docker or LXC/LXD images.