So yes, from Microsoft today:
Our new modern browser, Microsoft Edge, debuted over a year ago and today can be found on hundreds of millions of devices. Last August, we announced that Microsoft Edge Legacy desktop application support would end on March 9, 2021 as part of this transition to the new Microsoft Edge. Today, Microsoft Edge Legacy desktop application support officially ends, and it will no longer receive future security updates.
Additionally, we announced that the now out-of-support Microsoft Edge Legacy desktop application will be removed and replaced with the new Microsoft Edge when customers apply April’s Windows 10 cumulative monthly security update (or “B” release). For more details and an FAQ, please see our announcement blog post.
Having said that, web browsers are built on standards and they'll always work... to a degree. Edge will let anyone on, for example, a Lumia 950, browse most of the web for many years to come. The basics of http and https, CSS, JPG and PNG imaging, and so on, will mean that most sites can be navigated and enjoyed.
Most. But not all, since things are clearly starting to break. The issue is that the Web has been getting more complex for years. Back in the 1990s I was involved in the creation of a web browser for Psion and helped to detect and render simple HTML tags, including tables. So I know the basics of how web pages used to be created and in fact to this day I still hand craft my pages on stevelitchfield.com in raw HTML. But technology moves on, with CSS and HTML5, both of which Edge 'Legacy' (including under Windows 10 Mobile) handle pretty well.
In terms of Mobile, Web developers test their layouts and scripting through iOS and Android in their last couple of versions, with Safari and Chrome. And then possibly Firefox too, on a handful of modern smartphones. I can't blame them for not thinking of poor old Edge under Windows 10 Mobile, even if something as simple as a missing image really shouldn't happen - do we need scripting around a simple picture?
The main site quoted in AAWP comments was the BBC's News pages, for missing images, but I'm sure there are other examples. Comments welcome.
PS. Ironically, some have pointed out that the decade old Internet Explorer under Windows Phone 8.1 fares better and displays the missing images, but that will be because it's missing the scripting out completely rather then trying to interpret it and failing!