portions of libnisdb are likely dead code.
While working on #2023 I was being very careful because of a complicated amount of unusual (and really crazy) interposition that the Makefile stated was possible, and the code was carefully written to deal with. The makefile describes this interposition as being used by "numerous applications".
I can't find any. NIS+ is the only historical one I've been able to find (and it was not "numerous", it was "rpc.nisd" even then). It is likely that a lot of that code is actually part of NIS+, and dead.
It is likely that this will be challenging to prove.