Back in December 2018, Microsoft announced that it was replacing its Edge browser with a new Chromium-powered one, with the new browser being affectionately referred to by some as Chredge. After over a year, Edge Chromium was considered to be generally available in January 2020.
Now, support for Edge Legacy, which is powered by EdgeHTML, is ending today. The browser will no longer get any security updates moving forward. Starting next month, the legacy browser will actually be removed as part of the Patch Tuesday update, assuming that you're on Windows 10 version 1803 or higher. Microsoft hasn't outlined its plan for those on Long-Term Servicing Branch versions like 1507 or 1607, but presumably, you should be installing the browser manually. This change will also happen in the preview update that rolls out later on this month.
This change does not affect WebView, which is still supported. That's what's used in many hosted web apps and in parts of apps that need to view the web. Those apps aren't going to stop working, although you can always move your app to the Chromium-based WebView2 when you're ready.
After Edge Chromium hit general availability last January, the progress in its rollout was still very slow. It wasn't until June last year that anyone actually got it as an update. From January until then, GA only meant that you could install a stable version of the browser, rather than having to opt into the Beta, Dev, or Canary channels. Starting with Windows 10 version 20H2, which shipped in October, Microsoft actually shipped the new browser with Windows 10. But of course, it's businesses on older versions of the OS that are the issue being resolved next month.
Microsoft originally promised to be pretty much everywhere with Edge Chromium, expanding to platforms like macOS and older versions of Windows, with Linux support coming later. Indeed, the two goals that it originally had were to be everywhere Chromium is supported, and everywhere that Edge is supported. The latter was a bit more challenging, with some platforms being left on the back burner.
But you'll notice that those platforms are getting Edge Chromium updates now. For example, Microsoft's Windows 10 Team 2020 Update for Surface Hub contains the new browser, and the company is trying to get past some known issues to get that rolled out in time. Another platform only now getting Edge Chromium is Xbox.
If you're somehow still using Edge Legacy, you're considered to be secure until next month. After all, the final security update arrives today. You should absolutely be making preparations for when the browser is replaced next month.