You don't need one in the US for editorial photography (personal or news use) but you do for commercial use. Photography laws are not really logical, as a photographer, you own the photo. But, you can't sell or use it commercially without the persons consent who the image is of, as they retain the copyright to their "likeness".
I suppose as long as no one in the photo is identifiable, then no waiver needed, only if you can ascertain positive identification. Anyway, waivers would be for commercial use (which may include posting on social media, but it would be a long shot for the person in the photo to sue if it was just a selfie or family photo, not for sale or commercial use).