I just looked at the AMD_compressed_3DC_texture and AMD_compressed_ATC_texture specifications.
They introduce new undocumented compressed texture formats.
These specifications are very short, instead of technical detail they contain some marketing-blabala (“DC provides an ideal solution to the normal map compression problem. It provides up to 4:1 compression of normal maps, with image quality that is virtually indistinguishable from the uncompressed version. The technique is hardware accelerated, so the performance impact is minimal. […]”) and say that technical detail is not available (“The details of these formats is not disclosed […]”).
I always supposed that extension specifications should provide enough information to implement them.
These extensions are not in the OpenGL extension registry, but in the OpenGL ES extension registry instead.