User Configuration > Administrative Templates > Microsoft Office 2016 > Security Settings > Digital Signatures
User
HKCU\software\policies\microsoft\office\16.0\common\signatures
This policy setting allows you to configure Office 2016 applications to accept expired digital certificates during verification of digital signatures. If you enable or do not configure this policy setting, Office 2016 applications display digital signatures created with expired certificates as invalid. If you disable this policy setting, Office 2016 applications treat expired certificates as valid.