No, new features don't usually get backported to the stable branch.

It will ship with 1.13.9 (mainline) in 6 hours (at the time of writing).

For stable branch, you need to wait until the 1.13 milestone is completed, which according to their roadmap (https://trac.nginx.org/nginx/milestone/1.13) will be completed in 8 weeks.

It should be shipped with 1.13.9 according to the roadmap: https://trac.nginx.org/nginx/roadmap