vSAN6.6 just has been released to download.

vSAN 6.6 is a major new release that requires a full upgrade. We have to perform  following tasks to complete the upgrade to vSAN 6.6:

1. Upgrade the vCenter Server to vSphere 6.5.0d.
2. Upgrade the ESXi hosts to vSphere 6.5.0d.
3. Upgrade the vSAN on-disk format to version 5.0.

Note: Direct upgrade from vSphere 6.0 Update 3 to vSphere 6.5.0d and vSAN 6.6 is not supported.

Release notes are here.