Mentioned KB article is valid only for 5.1 version, not applicable for 6.5

Can you explain how SSO domain has been removed in the Web Client?

In general, you are unable to remove SSO domain (vsphere.local by default) in the Web Client - delete Identity Source button is greyed out.

You are able to remove localos domain and any custom domain (Active Directory).

To recreate localos or custom AD domain, you just need to click Add Identity Source button.

More information: Add a vCenter Single Sign-On Identity Source