It's already been well known that the legacy version of Microsoft Edge will no longer be supported later this year in March, but Microsoft has just announced its plans for what will happen once that time arrives. In a new blog post, the company detailed that the new Chromium version of Edge will officially replace and remove the legacy version of the Edge browser on April 13, 2021, by way of Windows Update.
According to Microsoft, the new Microsoft Edge will come to all PCs as part of the Windows 10 cumulative monthly security update—otherwise referred to as the Update Tuesday (or “B”) release—on April 13, 2021. Once this update is installed, the legacy version of the Microsoft Edge browser will be removed. The update will hit all the major versions of Windows 10, from the April 2018 Update onwards to the most recent Windows 10 October 2020 Update.
Note that if the new Microsoft Edge is already installed, applying this update will not reinstall the new Microsoft Edge and will only remove Microsoft Edge Legacy. Though you might be able to "Skip" the update, Microsoft recommends that you not skip it, as "windows cumulative monthly security updates provide critical updates to the Windows 10 operating system."
As for developers and apps that use EdgeHTML, Microsoft notes that the rendering engine for Microsoft Edge Legacy will continue to be supported. They also note that enterprise or business users who are depending on Legacy Edge's Kiosk mode should install the new Edge and set up its new Kiosk mode prior to April 13, 2021. This is to ensure no disruption in service.
It still remains unknown what will happen to legacy Edge in Windows Sandbox, however. We last noted that the legacy version of Edge is still present in Windows Sandbox, which is available in Windows 10 Pro.