The 4.12 oc client is missing the explanation of the --certificate-authority option which is nevertheless documented at official documentation page.

This is not happening with oc clients 4.13+.

    4.12.60

    oc adm prune images --help

    1. oc adm prune images --help
    2. check --certificate-authority is missing
    3. check Table 4 at official documentation https://docs.openshift.com/container-platform/4.12/applications/pruning-objects.html#pruning-images-manual_pruning-objects     

$ oc adm prune images --help 
[...]
Options:
    --all=true:
    Include images that were imported from external registries as candidates for pruning.  If pruned, all the
    mirrored objects associated with them will also be removed from the integrated registry.    --confirm=false:
    If true, specify that image pruning should proceed. Defaults to false, displaying what would be deleted but
    not actually deleting anything. Requires a valid route to the integrated container image registry (see
    --registry-url).

$ oc adm prune images --help 
[...]
Options:
    --all=true:
    Include images that were imported from external registries as candidates for pruning.  If pruned, all the
    mirrored objects associated with them will also be removed from the integrated registry.
    --certificate-authority='':
    The path to a certificate authority bundle to use when communicating with the managed container image
    registries. Defaults to the certificate authority data from the current user's config file. It cannot be used
    together with --force-insecure.
    --confirm=false:
    If true, specify that image pruning should proceed. Defaults to false, displaying what would be deleted but
    not actually deleting anything. Requires a valid route to the integrated container image registry (see
    --registry-url).