DTOO420 - The ability to store user passwords in Skype must be disabled.
Allows Microsoft Lync to store user passwords. If you enable this policy setting, Microsoft Lync can store a password on request from the user. If you disable this policy setting, Microsoft Lync cannot store a password. If you do not configure this policy setting and the user logs on to a domain, Microsoft Lync does not store the password. If you do not configure this policy setting and the user does not log on to a domain (for example, if the user logs on to a workgroup), Microsoft Lync can store the password. Note: You can configure this policy setting under both Computer Configuration and User Configuration, but the policy setting under Computer Configuration takes precedence.
Set the policy value for Computer Configuration -> Administrative Templates -> Skype for Business 2016 -> Microsoft Lync Feature Policies 'Allow storage of user passwords' to 'Disabled'.