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.
Is there any way to disable alphabetical sorting of network monitor's HTTP response headers?
From 53.x (IIRC) to 56.x, Firefox keeps the original order set by the server (like the one you see in the raw headers section or `curl -I`), and I like it that way.