Windows 10 Mobile is still dead, in case you still have some doubts about it. Last year, Microsoft completely stopped pushing new Insider builds for mobile following the release of the Fall Creators Update, so the writing was already on the wall. Yesterday, the Windows Insider team confirmed the obvious in a series of tweets: no new Windows 10 Mobile Insider builds will be released, despite some old tweets from Brandon Leblanc saying the contrary.
Plans change. Such is the nature of things.
— Jason Howard (@NorthFaceHiker) January 25, 2018
I just did. No mobile builds are coming. You won't see us talk about product strategy or comment on those things. That's not our team.
— Brandon LeBlanc (@brandonleblanc) January 25, 2018
Even if we won't see any new Insider builds for mobile, you may want to leave your Windows phones on the Fast Ring if you want to keep testing app updates for Microsoft apps. As for Windows 10 Mobile, it will be supported until December 2019 at least, which means that Microsoft will keep releasing monthly cumulative updates for quite some time. It's fine to keep using Windows 10 Mobile if you really like it, just be aware that the app situation won't get any better in the months to come.