It's opt-out. Here's the email they sent out to admins recently: You must exclude the extension before you install or update to a version of Office 365 ProPlus

It's opt-out. Here's the email they sent out to admins recently: You must exclude the extension before you install or update to a version of Office 365 ProPlus that installs the extension for Microsoft Search in Bing. Implementing the exclusion after the extension has been installed will not remove the extension. • For new installations of Office 365 ProPlus, the Office Deployment Tool may be the best method, as outlined in this support document • For existing installations of Office 365 ProPlus, modifying the Group Policy may be best. Enable the policy setting Don't install extension for Microsoft Search in Bing, which makes Bing the default the search engine. • If you use Microsoft Endpoint Configuration Manager (current branch), from the Features section, set Microsoft Search as default to the Off position. • If you use Microsoft Intune to deploy Office 365 ProPlus, clear the check box Microsoft Search as default on the Configure App Suite pane.