Do you support Salesforce Classic?

We have followed Salesforce direction and built Mailchimp for Salesforce to be Lightning only. This is largely due to Salesforce's investment in the Lightning UI, releasing features that only work there. It has meant that we have been able to build some great features that wouldn't be available in classic. Salesforce hasn't given an official date when it will retire classic, but it's now best practice to develop in Lightning.