Nether is no longer a mining dimension, GT6 ancient debris is small and bedrock ore. I’m assuming ancient debris is from GT6. So… to get ancient debris from small ores, one would have to mine massive amounts of netherrack. To get from bedrock vein… either luck, or long search.
I once tried to find it, found (almost) every other bedrock vein, but not ancient debris. No, nether in that world was not generated before ancient debris bedrock veins existed. Just luck issue.
In current world I have witchery, which allows to summon wither while having just 1 wither skeleton skull.