Public service ads (PSAs) are being retired in AdSense and will no longer be a backup ad option. Google is now trying to figure out new options.
If you're a website publisher and want to avoid having a blank space where a PSA used to appear, you can select and edit your ad unit on the "My Ads" page (for the old interface, edit ad settings by going to "My Account," and to to "Manage Ads").
Those who have chosen to show non-Google ads from another URL or a solid color won't be affected. PSAs in AdSense were shown on pages when a relevant paid ad was unavailable or the site hadn't yet been verified.
Google will continue supporting its non-profit partners with free advertising through Google Grants. Ads in the Google Grants program won't be affected.