I was surprised to discover the man pages having entries for two conflicting variants of readdir.
in READDIR(2), it specifically states you do not want to use it:
This is not the function you are interested in. Look at readdir(3) for the POSIX conforming C library interface. This page documents the bare kernel system call interface, which is superseded by getdents(2).
I understand a function may become deprecated when another function comes along and does its job better, but I am not familiar with other cases of a userspace function coming in and replacing a kernel function of the same name. Is there a known reason it was chosen to go this route rather than coming up with a new function name (as the man page mentions getdents did when superseding readdir).
The programming interface, POSIX, is stable. You don't just go replacing functions in it unnecessarily because you want to implement the backend more efficiently. The Linux syscall readdir never implemented the readdir function because it has the wrong signature; it was an old, inefficient backend for implementing the readdir function. When a better backend came along, it was obsolete.