You can sometimes find older customized ISOs by entering your server's service tag on the Dell Support site [4].
VMware ESXi is a Type-1 hypervisor, meaning it installs directly on server hardware (bare metal) without requiring an underlying operating system. Version 6.0 was a milestone release for VMware, introducing significant features that modernized data centers at the time. Vmware Esxi 6.0 Download Iso