Yes it happens often from times to times, I think EPIC should care more about us, and let the UE3 Reference forever online, because it is a precious source of knowledge with many information about one of the most fantastic game engines of all times. Even though UE4 is the natural evolution of UE3, however UE3 has a great importance to the game industry in general, it contributed shipping many of the most famous game franchises.
About the UDN getting offline, whenever it happens, I use this site to get a cached version (google cache):
An Epic staff member did respond to my post and the online docs are back up and running. I really appreciate that they’re still looking out for us (even if you do have to post on the UE4 forums to get noticed).