[1, 3]. It existed to let users know the app wasn't an official Google product, preventing phishing and keeping the ecosystem safe [1].
While there is no direct "off" switch in the settings to remove the branding banner for free, you can use technical workarounds to hide it from your end users. Google includes this banner as a security measure to alert users that the application was not created by Google itself. Workaround 1: Embedding via Iframe [1, 3]
If you use a paid Google Workspace (formerly G Suite) account, the banner behavior changes: Internal Users Google includes this banner as a security measure
For a truly "free" experience that looks professional, focus on clean UI design within your HtmlService files. A well-designed, functional application will often earn the user's trust regardless of the small disclaimer at the bottom of the page. [1, 3]