Wizards of the Coast have moved from providing previews of the DMG to providing “goodies” for DMs. The Magic Items By Rarity listing is a 3 page long supplement and the first resource of a set of supplement resources they plan to provide. Additional supplements will be released in the new year, including a Random Encounter Tables. An example and how to build a random encounter table is already provided on page 87 of the Dungeon Master’s Guide with an example.
Visit the Dungeon Master’s Guide page at Wizards of the Coast