Logging doc has old contents https://docs.redhat.com/en/documentation/openshift_container_platform/4.21/html-single/logging/index 

We've already fixed this in 4.18, 4.19, 4.20. Apply same change to 4.21.

Logging doc is moved to https://docs.redhat.com/en/documentation/red_hat_openshift_logging/

This issue also exists in future release 4.22. This Jira will also be used to fix that.
The fix is already present in main. For the future, the root cause as to why new release branches contain outdated code even though this has been fixed in main needs to be identified.