GUI editor strip trailing whitespace

Is the fix associated with this Fixed bug included in Octave 6.4?

https://savannah.gnu.org/bugs/?57860

It appears that the bug was closed in February of 2020 but I can’t find that setting in the Editor preferences.

this was pushed to the default branch, not the stable branch, since it introduced ‘risky’ behavior (deleting part of someone’s file). As such, it should be part of Octave 7 when it is released soon.

@nrjank is basically right. That change went on the default branch and will be part of Octave 7.x. But I don’t think the reason for it going there was the “risky” nature of that change.
Changes on the stable branch are basically limited to bug fixes and documentation changes. Sometimes it’s difficult to draw a line between what is a bug fix and what is a new feature. But in this case, it’s pretty clear that it is a new feature.
Having said that, it isn’t a hard rule that new features never land on the stable branch. Especially in the phase just after merging default to stable, we landed new features also on the stable branch. That decision was made on a per-case basis. (E.g., low-impact feature with little risk of breaking anything or features considered “very important” could land on stable.)
For this particular feature, it was decided to ride on the default branch. Reasons for that decision are given in the bug report.