Coal ore still generates as deepslate

If deepslate coal ore shouldn't generate naturally because it can't generate below y=0, then why did I see deepslate coal ore generated in front of me?

I don't know if this was intentional or an issue, but it did say: "While they will not generate in the world by default, Deepslate versions of copper, emerald and coal ores have been added for use by creators of maps and data packs".