Who is impacted by this change?
Any partner or customer using Web-to-Case (W2C), Web-to-Lead (W2L) functionalities on their website, or using www.salesforce.com as an API endpoint. 

Why the change? 
To improve the performance of the Web-to-Case (W2C) and Web-to-Lead (...

As previously communicated by Salesforce (Default Certificate to Retire on August 7, 2017) the proxy.salesforce.com default certificate has been retired due to its expiration and for security best practices. If your Salesforce org uses this certificate (the “Default Ce...