Similar to the bullet (.uno:DefaultBullet) and numbering (.uno:DefaultNumbering) list split buttons found in the formatting toolbar, we need a similar one for outline lists, to include entries found in the Bullets and Numbering dialog's Outline tab.
Thanks Maxim. You can add it as a hidden entry in the formatting toolbar after numbering, for those who want to enable it.
Maxim Monastirsky committed a patch related to this issue.
It has been pushed to "master":
tdf#95403 Add Outline button
It will be available in 5.1.0.
The patch should be included in the daily builds available at
http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.