From the Store description:
We’ve been working hard and feel confident enough to open up our Windows Phone Beta to all the patient Windows Phone users who've been clamoring for it. But this is a beta — not the finished version, so it has some rough edges and quirky traits unbecoming of a proper Slack application. We humbly ask only one thing: please give us your honest feedback so that we can make your Windows Phone experience great.
As I say, this has been out for almost two years, yet is still in 'beta' and updates have been coming along bi-monthly, yet there are still some yawning gaps in functionality, at least when compared to the full Desktop Slack experience:
- there's no voice calling
- there's no screen sharing
- file sharing is limited to sending images
- there's zero integration with other Windows Phone applications
- there's no dark theme
Having said all that, the basics of inter-team chat do work as advertised, and in a UWP-like interface (hey, at least it's not based on Silverlight!), here's Slack (Beta) in action:
Not bad then, and it's all free, of course, though it's a million miles from the Slack functionality on other platforms. The Windows 10 Slack app is in the Store but derived via Project Centennial from a native Windows x86 application, so it seems that Slack doesn't want to go down the road of a full UWP application for all form factors.
Still, it's a lot better than nothing. You can grab Slack (Beta) in the Store here.