This article provides information on which vSphere/vSAN versions are supported and not supported for vSAN 6.6 upgrade.
The underlying release for vSAN 6.6 is vSphere 6.5.0d which is a patch release for vSphere 6.5. Before upgrading to vSAN 6.6, please see VMware Product Interoperability Matrices to ensure your current vSAN version is supported for upgrade.
Upgrading to vSAN 6.6 from most prior versions is supported. However, upgrading to vSAN 6.6 is not supported when you are using vSAN on vSphere 6.0 Update 3. For more information on vSphere upgrade policies, see VMware vSphere Upgrade Policies (2149713).
“For the purpose of upgrade supportability, any patch release is treated the same as its parent release”. The parent release of vSAN 6.6 (patch release vSphere 6.5.0d), is vSphere 6.5 and as shown by VMware Product Interoperability Matrices. Upgrade from 6.0 Update 3 to vSphere 6.5 (and hence vSAN 6.5) is NOT supported. Therefore, an upgrade from 6.0 Update 3 to vSphere 6.50.d (and hence vSAN 6.6) is NOT supported.
Note: vSAN 6.0 Update 3 users who want to avail of the new vSAN 6.6 features will need to wait for a future update release. Please contact VMware support if you need further details.