Starting with vSphere 4.1, there is support for both IPv4 and IPv6; although the IPv6 support was initially disabled by default. With vSphere 5.1, IPv6 has become enabled by default for VMkernel traffic.
It is better to keep it vSphere 6.5 enabled, also because there is a bug in ESXi 6.5 (not fixed yet with Update 1) where the ESXi host 6.5 may fail with a Purple Screen of Death (PSOD) when IPv6 is disabled. For more information, see KB 2150794—ESXi 6.5 host fails with PSOD when IPV6 is disabled at https://kb.vmware.com/s/article/2150794.
For guest OS, both on Linux and Windows systems, it becomes very difficult to disable it, and Microsoft itself does not recommend disabling IPv6. We do not recommend that you disable IPv6 or its components, or some Windows components may not function. Find more information at https://support.microsoft.com/en-us/kb/929852.