What condition would prevent an administrator from creating a new VMFS3 datastore on an ESXi 6.x host using the vSphere Web Client?

Starting with ESXi 6.0, VMware dropped support for creating VMFS3 datastores because VMFS5 became the required format for new storage provisioning. The vSphere Web Client on an ESXi 6.x host will not present VMFS3 as an option when formatting a new datastore. This architectural decision was made to enforce adoption of VMFS5 improvements such as unified block size and GPT partition tables.

The VMFS3 kernel module being unloaded is not a standard or documented condition on ESXi 6.x; the restriction is a product-level policy, not a missing module.

ESXi 6.x hosts can still mount and access existing legacy VMFS3 datastores that were created on older hosts, so mounting is not universally prevented.

VMFS3 datastore incompatibility with VMs is not the stated restriction; the issue is that new VMFS3 datastores cannot be created, not that existing ones are incompatible with VM files.