If your activity history is synced across your devices through your Microsoft account (MSA), you will no longer be able to upload new activity to Timeline. Accounts connected to AAD will not be affected. To view web history, Edge and other browsers have the option to go back to recent web activity. You can also view recently used files using OneDrive and Office.
Microsoft has since updated the post to clarify that the timeline and history of related activities will still be part of Windows 10, but if you’re using a Microsoft account, it will only be available locally on that machine. What is behind the change is not entirely clear, but as Thurrott points out, it also lost Android sync at the end of last year when Microsoft killed its launcher on the platform, so the potential for loss remains high.
If you are not in the Insider beta program, a Microsoft support page mentions that cross-device syncing will go away for you in June, so anyone who uses it will have a few months to prepare. The timeline was the Microsoft’s Fall 2018 Update “Star” for Windows 10 (after a delay it was removed from the previous Creator Update) and while it will still be remember everything you did, this will no longer save this information in the cloud.