By default, your content may be used in an anonymized form to help improve Napkin's AI models. You're always in control though. You can opt out anytime from Account Settings, under the Data Privacy tab, by turning off "Use my content for product improvements." This applies whether you're using Visuals or Napkin Slides.
Who can see your presentation
Your presentations are private by default. Only you can see them unless you choose to share them. See Sharing Your Presentation for how to manage permissions.