According to the dev in a blog post, "It means that there will be no new content created for AOEO. There is a small amount of content still in the pipeline that will be released in the next few weeks, but that is all. No new features will be added. Most significantly, no new civilizations will be released; the six existing ones are the complete and final set."
However, Age of Empires will go on as before. "You can still do all the things you do today; we’re not taking away anything you already have. There will be no new civilizations or features, that’s all," the post reassured.
However, the dev warns that patches to correct bugs will be slower.
"Fixing any remaining bugs and addressing balance issues will be slower and, frankly, more difficult for the team. Some may, in fact, be unfixable. We will be watching carefully in case any critical bugs appear," stated the blog.
The last premium DLC released for Age of Empires Online were the back in October 2012.