In multiple files, nether fossils are listed with other structures that can be located through /locate. It is also assigned a string that the command would accept, Nether_Fossil. The other structures listed here are only /locate-able structures.

I'm not sure if this information would differ based on the program used to deobfuscate the code, but the file names where I found this were Feature.java and StructureFeatureIO.java.

This seems to imply nether fossils were intended to be compatible with /locate, or that this was leftover on accident.