Why is Microsoft doing this?
Office for Mac 2011, like most Microsoft products, has a support lifecycle during which we provide new features, bug fixes, security fixes, and so on. This lifecycle typically lasts for 5 years from the date of the product’s initial release.
What are my options?
Although you'll still be able to use Office for Mac 2011, you might want to upgrade to a newer version of Office so you can stay up to date with all the latest features, patches, and security updates.
Upgrade options include:
Upgrading to Office 365, the subscription version of Office that comes with Word, Excel, PowerPoint, and other apps, depending on the plan you choose. With Office 365, the apps are available on your computer and via browser; one subscription enables you to use them on both your Mac and PC.
Upgrading to Office 2016 for Mac. Office 2016 for Mac is a one-time purchase (not a subscription) for installation on one Mac only.