It is a fact that Wizards of the Coast did at some point remove all content for 4th edition D&D from its site when 5th Edition had come around. Since then, there is no mention of 4th edition on the page; only 5e content remains.
Such an act, if deliberate, is in history sciences often called Damnatio Memoriae, the condemnation of memories, and then an attempt to remove a person from history. However, it might also be just spring cleaning with the 5th edition taking center stage. In any way, some highly regarded content fell into nirvana and is lost.
Some of this content was archived by the Wayback Machine, but I find it hard to pin down the exact date of this massive purge of content.
When did Wizards of the Coast purge D&D 4th Edition content from its website?