If you are one of the users using Microsoft Office 365, you would have come across this small feature in PowerPoint 365, especially on the Windows desktop clients, which shows up a pop-up when you have more than 25 unused slide masters in your PowerPoint slide.
How to clean up unused PowerPoint master slides in PowerPoint?
Your message will look something like this.
“PowerPoint found a large number of unused slides. Review Slide Masters by going View > Slide Master PowerPoint can also remove unused Slide Masters for you Perform Clean-up (Recommended ? )”.
You can accept or decline to cleanup.