The End of General Support (EOGS) for vSphere 5.5 and vSAN 5.5 is September 19th, 2018. To maintain full level of Support and Subscription Services after this date, customers will need to upgrade to vSphere 6.5.
Upgrading to vSphere delivers the latest vSAN release and capabilities. vCloud Suite 5 and vSphere with Operations Management (vSOM) customers running vSphere 5.5 are also recommended to upgrade to vSphere 6.5. For more information about the VMware vSphere Upgrade, please visit: https://www.vmware.com/products/vsphere/upgrade-center.html