The vast majority of client webinars I moderate feature PowerPoint slides with a footer or disclaimer slide declaring the materials to be confidential. They usually aren't. And there is a real, legal reason you shouldn't label them that way.
Confidential has a legal meaning. When parties sign a business confidentiality or non-disclosure agreement, there is a clause that covers materials designated as confidential. There are penalties for disclosing or distributing those designated materials.
Consider a situation where your company has to go to court, claiming that someone has violated your confidentiality agreement and has disclosed your private materials. If that party can demonstrate that you just slap "confidential" on all your public presentations, they could claim reasonable precedent for believing that the term has no validity or distinctive power in identifying what information is truly private.
A public webinar NEVER features confidential information. Always assume that everything you display and say in a public webinar instantly becomes part of the public domain and is discoverable by competitors. Confidential materials are materials you only show to people who have signed a nondisclosure or confidentiality agreement with you. Internal employee presentations, partner training, and some customer communications may fall under this label. Marketing, lead generation, and public outreach webinars never do.
What should you write on your slides instead? You can (and should) claim copyright ownership so others can't legally reuse your work. You can throw in standard terminology that attempts to discourage redistribution, but you should keep in mind that it has no actual legal weight. An attendee who comes to your webinar is not bound by any statements you might unilaterally make about how the material "is intended for the recipient only and is not to be redistributed." But at least it won't get you in legal trouble later.
Take the time to make the distinction. Most of your webinars and presentations are not confidential. Make sure you can clearly identify the ones that are!