We are assuming that we are talking to a recursive name server although this is not what this feature is designed for. It should be changed to stub-zone.
https://git.ipfire.org/?p=people/ms/ipfire-2.x.git;a=commitdiff;h=209827ab9db312de972c9c9b9e7b1c0f92948320
Has this patch ever made it into IPFire 2.x?
No, this has not been submitted to the list, yet. I have tried to get more people engaged into the process to work on the new DNS features, but unfortunately nothing has happened. So this will sleep in a branch now and probably will never find its way into the distribution.
I am resetting it back to ASSIGNED, then.
https://git.ipfire.org/?p=ipfire-2.x.git;a=commitdiff;h=15cf79e3b864bef46d32b08e3b381a654e00950e This has been merged in Jan 2020.