Testing with 6 worker 3 master node vsphere cluster, with local devices for LSO. OCP 4.9.0-0.nightly-2021-09-18-052905 (k8s v1.22.0-rc.0+af080cb) LSO 4.9.0-202109132154 ODF 4.9.0-142.ci And I can confirm, that it's possible to select arbiter mode in the UI when the arbiter requirements are met. I haven't tested whether it's actually possible to deploy a cluster successfully, as this is blocked by BZ BZ 2000301.
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 (Moderate: OpenShift Container Platform 4.9.0 bug fix and security update), 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/RHSA-2021:3759