Issue reported to the KDE folks with KDE Bug 366177:
<https://bugs.kde.org/show_bug.cgi?id=366177>
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Summary:
The check box "Only owner can rename and delete folder content" does not set a directory's "sticky bit".

If the directory's "sticky bit" is set from the CLI, Dolphin indicates the "T" in the "Permissions" field and, if the check box status is not changed, does not change the directory's "sticky bit".

If however the check box is cleared, or cleared and then re-enabled, on clicking the "OK" button, the directory's "sticky bit" is cleared.