Once you’ve been working with slide masters for a while, it can start to get a little confusing and cluttered. Luckily, you can rename, delete, and preserve slide masters and layouts.
Sometimes PowerPoint may try to clean itself up and delete any slide masters that aren't currently being used in a presentation. To prevent that from happening, preserve your slide masters.
From Slide Master view, select a slide master.
Click the Preserve button on the Slide Master tab.
Right-click the slide master and select Preserve Master.
A tiny pin symbol appears next to the slide master, so you know that it won’t be deleted.