Yeah, the Edge codebase change and its resultant loss of that UWP EPUB reader was listed as one of the key factors in that much publicized case of Microsoft entirely shutting down their Ebook store.

In early Windows 8 history the UWP PDF Reader (that went on to also be the EPUB Reader) was available as a separate app in the Windows 8 Store just called "Reader", before it merged into Edge. I wish they would spin it back out into a new Reader app again, just to keep a useful extra option around for PDF and EPUB files, but it sounds like the code shares too much with the EdgeHTML Renderer they are entirely spinning down for them to do that, which is a shame.

I wouldn't mind a reader app to keep using the old Edge engine. It hardly needs new web standards support anyway.