Minecraft wiki entry for Old_Customized, associated text: Customized, retroactively labelled Old Customized by later game versions, was a world type that replaced the normal varied terrain of the Overworld with custom features. Because, as of snapshot 18w06a, it was removed completely due to the rewrite of the world generation, it was then replaced by a new, easy-to-customize but considerably more limited substitute world type called Buffet. Custom worlds returned in 20w21a, though they are completely different; this version also renamed this world type to Old Customized.