I can't find Ancient Debris in the Nether

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I think it's a bug ! The Ancient Debris can‘t generate normally in the Nether.

Could you provide a world save where Ancient Debris does not generate?

Yes , of course. But the zip file size exceeds the limit.

You can use a file sharing site like OneDrive, GoogleDrive, etc.

https://drive.google.com/file/d/1Cn0rXpLBK0m7cWLKKk79A8SwW-XpsUcq/view?usp=sharing

Would this be because you are looking for ancient debris in chunks loaded before 1.16? 

May be.But I remember  seeing the  ancient debris in 1.16.1.

You need to make sure that you are destroying newly loaded chunks (after 1.16) and that you are doing it at around the same levels you find diamonds at. 

@@unknown I'm not able to import the world save you've provided, it keeps saying "import failed." However, I can see from the files that you have a lot of add-ons applied to the world. We can only accept bug reports for issues that occur in vanilla Minecraft, sorry.

What others have said about searching in chunks generated in 1.16.0 or later is correct. When I load newly-generated chunks in vanilla Minecraft in 1.16.20, I can find ancient debris.