IBM Z has been supported since 4.2.z and Power since 4.3.z, so should not be listed as unsupported in clidownloads.
Now the text is correct: "Download oc for Linux for IBM Z" Verified on 4.6.0-0.nightly-2020-09-09-173545
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory (OpenShift Container Platform 4.6 GA Images), and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://access.redhat.com/errata/RHBA-2020:4196