Our installation documentation has no mention of this CIDR (only the join subnet) and we can't expect customers to read all other documents prior to install the clusters as we can see in the example below:

https://docs.openshift.com/container-platform/4.15/installing/installing_bare_metal/installing-bare-metal-network-customizations.html

Version-Release number of selected component (if applicable):
OCP 4.14 and 4.15

every time