Remove This | Application Was Created By A Google Apps Script User

Since you are hosting the environment, Google does not inject any security banners. Why Does Google Put It There?

If a branded, professional experience without such disclaimers is critical, consider: Since you are hosting the environment, Google does

No. The warning is purely informational. Your script continues to run perfectly. Users can click through and use the app normally. However: The warning is purely informational

The banner is mandatory and cannot be removed. However: The banner is mandatory and cannot be removed

Use a Google Cloud project with proper OAuth branding

This will revoke the script's permission to access your account, effectively stopping it from running.

If your app truly needs full, unrestricted access to user emails and Google Drive and serves the general public, removing the warning might be impossible without a security assessment. In that case, consider building a native web app using Cloud Run or Firebase instead of Apps Script.