On a mobile-related talk page, user WikiLeon said regarding mobile Windows "I noticed the app has been deprecated since 4 months ago. I think it's time for WMF to take it off the Windows Store," 
That spawned an email thread, in which Brion said "I dug up the last mails from Raj & co at Microsoft, from about 4 weeks ago; they have a demo Win10 app that wraps the Wikipedia mobile interface in a web view that's fairly pretty, with an integrated back button. I tested it briefly on desktop/tablet, but it should work on phone as well if I understand how this stuff works."
And Toby followed with "We actually have a plan to publish the new app with WP Cascadia! ... Make sure to specify [in the phab task] Microsoft wrote the app. We agreed with them that we couldn't own it. "
Adam suggested that I create the task in the Reading-Admin project, so I have. And with that, I will leave y'all to it. You know better than me what steps should actually be taking to resolve this situation.