Comics Heating Up were the first official site to break the news besides some folk on Twitter chatting about it.
I'm legitimately impressed this was kept a secret all the way to when the books started shipping-out considering how rarely something is ever kept quiet in this era of everything leaking. It was cleverly hidden too, with solicitations for later issues that never were actually coming out appearing in, "Previews," magazine so that folk were none the wiser. Now major news sites are talking about this as it is a big deal for this series to end--even if the main show and countless spin-offs will most likely keep chugging along for a good chunk of years. I never read the series too much or watched the show, but it is clear, "The Walking Dead," became quite the cultural phenomenon and the series surprise conclusion is quite the shocking twist, so I tip my hat to Kirkman and friends for keeping this under wraps.