When upgrading a standalone vCenter Single Sign-On Server version 5.5 to version 6.5, which will be the final deployment type?

VMware does support this upgrade path - the standalone SSO server is a recognized source topology for migration to vSphere 6.5.

In vSphere 6.x, the vCenter Single Sign-On service was consolidated into the Platform Services Controller (PSC). A standalone SSO 5.5 node has no embedded vCenter Server, so after upgrade it maps to the external PSC deployment model, continuing to provide SSO, licensing, and lookup services separately from vCenter Server.

An embedded PSC only results when vCenter Server and SSO are co-located on the same node in 5.5; a standalone SSO server has no bundled vCenter component.

A standalone vCenter Server deployment refers to a vCenter instance with no external SSO/PSC, which is not the result of upgrading an SSO-only appliance.