It's not very useful for regular users outside of private browsing - the WDAG windows are unable to persist any state at all since their container is discarded

It's not very useful for regular users outside of private browsing - the WDAG windows are unable to persist any state at all since their container is discarded at the end of the browsing session to thwart malware persistence. It does sound like an interesting capability for private browsing, but existing mechanisms mostly cover that. Even if WDAG applied to private Windows, ordinary users aren't so likely to open private browser windows just to check a link from an email. I expect that if you had a copy of Win10 Enterprise you could configure the feature yourself for added security in paranoid cases (e.g. journalist covering abusive regimes who might be targeted by state-level malware).