Windows 10 Sun Valley Update drops support for cross-device sync in Windows Timeline. Timeline lets you see a timeline of activities that you performed in Windows 10 including webpages you visited, documents you created/opened, photos you added, and more. Microsoft notes that Timeline activity history sync support is no longer available for those with Microsoft account. It will still work if you use AAD-connected accounts, but you can no longer sync activities across devices when they’re connected to Microsoft account, which means Timeline is not being discontinued.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10s-multitasking-feature-is-getting-downgraded/