How to get legacy Edge back on Windows 10
The new Chromium-based Edge browser is excellent in many ways and – despite some occasional issues – it's as ready as Chrome is to be your default browser.
Installing the 20H2 version of Windows 10 will switch your default browser to the new Edge automatically, because the 'legacy' version of Edge isn't included in 20H2. That's because legacy Edge will no longer be supported after March 2021: so enterprises that aren't ready to migrate to the new Edge will have to postpone updating to the upcoming version of Windows until they can adopt the new browser at the same time. (That also means none of the tips for getting legacy Edge back work in recent Insider builds of Windows because they're 20H2 builds without legacy Edge in.)
Versions 1803 and later of Windows 10 get automatically updated to the new Edge through Windows Update (unless you use Windows Update for Business). That's a fairly seamless experience, copying across your favourites and passwords, and even reopening any tabs you had open in Edge before you restarted your PC (upgrading to 20H2 doesn't reopen tabs from legacy Edge, probably because the old browser isn't there to get a list of tabs from).
Windows 10
But if you're a developer or IT admin supporting users on older versions of Windows 10, you might still need access to the 'legacy' version of Edge (or you might prefer the thumbnail tab previews to having dozens of tabs show up in Alt-Tab when you need to find one browser tab out of the dozens you have open).
SEE: Windows 10 Start menu hacks (TechRepublic Premium)
It's easiest to keep legacy Edge available before the new Edge gets automatically installed, by running the Chromium Edge Blocker Toolkit (at an admin command prompt).
This creates the registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\EdgeUpdate and the DWORD DoNotUpdateToEdgeWithChromium with the value of 1. Now you can install the stable, beta or canary version of the new Edge as you prefer.
But if you missed a machine or (as happened to me), you ran the Blocker Toolkit on a new PC that rebooted with a blue screen bugcheck before the registry key was created, you can still get legacy Edge back – but it's much more complicated, and you may prefer to roll back your system to before the new Edge was installed, run the Blocker Toolkit and then reinstall the system updates.
If you use Group Policy, you can enable the Allow Microsoft Edge Side by Side browser experience group policy. Or you can create the DWORD Allowsxs, again in Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\EdgeUpdate, and set the value to 1.
Next you need to run the Edge installer again (ignore the warnings that you already have it installed). That puts Microsoft Edge Legacy on the Start menu; if you had it pinned to the taskbar or Start menu, you have to do it again because the new Edge browser will have taken over those pins.
If you want to make legacy Edge your default browser you can do that from Settings > Apps > Default apps, but the browser you pick there is Microsoft Edge rather than Microsoft Edge Legacy and that doesn't open from the Microsoft Edge Legacy icon, so when you open a link in another application, you (rather confusingly) end up with two legacy Edge browser icons. Keep the Microsoft Edge icon (blue on a black background) pinned rather than the Microsoft Edge Legacy (white on a blue background) icon, because the latter always opens with the Microsoft Start page full of cheesy news stories rather than any tabs.
When you make legacy Edge the default browser, that isn't the same legacy Edge that you find in the Start menu.
Even after turning on Side by Side and repinning legacy Edge and setting it as the default, Edgium can take over again when you install a system update. We're investigating with Microsoft if there's a bug that blocks Side by Side in the KB4568831 Cumulative Update. In the short term, if you need to get into legacy Edge to find something in your history or to use Flash on Windows on Arm so you can use a work site that requires it (Edgium doesn't support Flash on an Arm device like the Surface Pro X), there is a short term fix in the form of a registry key you can delete. It's not a permanent solution because it will get recreated every time the stable version of Edgium gets updated, but if you're stuck right now, it will unblock you.
SEE: Windows 10 2004 issues: Now browser bugs hit – Edge startup launches, Chrome sign-outs
In the Registry Editor, open the Computer\HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\EdgeUpdate\ClientState\{56EB18F8-B008-4CBD-B6D2-8C97FE7E9062} key, and delete the BrowserReplacement key value. Now the legacy Edge icon will go back to opening legacy Edge (although you will probably have to change your browser default away from the new Edge as well).
It's a nudge nudge nudge world
Get used to being reminded that you're going against the flow though, because Microsoft wants to send a clear signal that you're clinging to the past. Despite the design philosophy of Microsoft 365 being to create "experiences that encompass your wellbeing" Microsoft is as invested in 'nudge theory' as the UK government.
When Microsoft says "because nobody knows your external circumstances or inner emotional state better than you, achievement needs to happen on your terms to be sustainable" and asks "what extraneous visuals can we remove so your content, not the UI, becomes the sole focus?" that doesn't mean that Microsoft web sites aren't going to nag you to set the new Edge to be your default browser even if you tell legacy Edge to stop asking, just as Outlook will nag you to install mobile Outlook when you just wanted to read your email.
Even after you tell legacy Edge that you don't want to change the default to the new Edge, Microsoft sites will nudge you into changing, just as Outlook will pop up an ad for the mobile version.
This is what Google has been doing for years when you visit Google properties using a Microsoft browser (even one built on Chromium) and it's not that different from Apple wanting to keep customers entirely in its ecosystem, so maybe it's unfair to criticise Microsoft for doing the same when the issues with legacy Edge are more about web developers not supporting it than any inherent issues with the browser – even if it does seem to contradict its product principles.