How is Strategy Office functionality affected after the code signing certificate expires?
The Strategy Office plugin is code signed by a Certificate Authority (CA). Microsoft Office requires plugins to be signed; otherwise Microsoft Office security settings may prevent the plugin from loading on start.
Code signing certificates have a limited validity, in the case of Strategy Office the code signing certificate is valid for 3 years:

The code signing certificate must be valid at the time the code was signed – i.e. at the time Strategy Office code is compiled to the plugin. There is no impact on the functioning of the Strategy Office plugin after the validity period of the code signing certificate expires. Microsoft Office will perform checks if a plugin is code signed and if the certificate was valid at the time the code was signed.
Therefore, no further action is required.