A página man em man7.org e no Debian tem uma descrição mais útil :
/proc/sys/fs/inode-max
(only present until Linux 2.2)This file contains the maximum number of in-memory inodes. This value should be 3-4 times larger than the value in file-max, since stdin, stdout and network sockets also need an inode to handle them. When you regularly run out of inodes, you need to increase this value.
Starting with Linux 2.4, there is no longer a static limit on the number of inodes, and this file is removed.
Com base na última frase, não está lá, pois não é necessário.