Enabling this setting would place an item in the chestplate slot before the helmet slot:
(the screenshot has the armor items sorted from bottom to top, with boots and leggings showing first)

What, It is bug. It should be sorted originally.
What is your version, modloader?

This is a feature suggestion, there is no bug.

Not sorted that is bug.
It's sorted in fabric 1.17.

As with #12, I didn't find any issues.

This GitHub issue and #12 are both feature suggestions/requests. They're not bug reports

It means that you already have that feature.
If those features don't work, it's a bug.

If they're already features, how do I enable them?
I didn't see them in the config

There are no config. It just default features.
That's why I asked for the version.

2.5, Fabric 1.17.1.

Based on your screenshot, it is not a feature enabled by default. If it was, then your elytra would be inbetween the netherite and iron gear on your armor bar.
This is a feature request for a feature that isn't implemented currently.

As for #12, the current system puts your gear onto the armor bar top to bottom.
You perfectly showed that in your screenshot: your iron helmet is on the left of the occupied space on the armor bar, not at the end of it.
That feature request is for asking a way to toggle it between having it show bottom to top, instead of top to bottom.
Sorting bottom to top would place your boots on the left of the occupied space on the armor bar, not at the end of it.

Added in 2.6 version. thanks for your suggestion too.