From the Itch.io - Content creator quality guidelines:

Avoid obtrusive advertisements or third-party logins

We prefer to see projects that have no advertisements but we realize that this isn’t possible for all developers. If you have advertisements, please don’t make them obtrusive or misleading. Avoid sponsored installers, or ads that prevent someone from playing the game without performing some action.

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Any projects that contain malware, spyware or adware are prohibited and will be banned from our platform. This includes any installers that put advertisements on the user’s operating system or browser.