From The Document Foundation Wiki
Where do I find the master slides in a presentation?
To view the master slides in Impress:
- Make sure the Sidebar is visible: ▸ .
- If the Master Slides deck is not visible in the Sidebar:
- Click the Sidebar Settings button at the top of the Sidebar.
- Under Customization, check Master Slides.
- Click the Master Slides button in the Sidebar.
The master slides are stored in the presentation.