BSON_CONTEXT_DISABLE_HOST_CACHE is still mentioned in the docs for bson_context_t and some code doc blocks, but it is no longer used. It is defined in bson-types.h, but I expect that must remain for BC until libbson 2.0.

I assume this flag became obsolete when when the ObjectID spec was implemented in CDRIVER-2771. Currently, the hostname is used to seed the RNG used for generating the 5-byte component of an ObjectID, but that has nothing to do with the flag.