The isolinux/initrd.img on a standard RHEL 8.10 ISO image definitely includes /usr/bin/rmdir - I checked.
Without knowing what the customization in your custom RHEL 8.10 ISO is trying to achieve, it is hard to say anything definite, but it seems to me that something has probably gone wrong in the earlier customization process. Not having a basic command like rmdir available during the installation process might cause some number of package scripts to fail, making the customized ISO unusable as a general-purpose installer. But maybe that is not the goal in this case?
I would recommend contacting whoever made that initial customization, and reporting the problem. It's probably easier to correct the earlier error that caused `rmdir´rmdir to be not available than trying to further cut out functionality in an effort to live without it.