Solid Explorer Beta was released a few months ago, and was instantly a fabulous file management app. It's clever use of swipable columns and fragments makes it easy to move things around. If you've noticed a distinct lack of updates as of late, there's a reason for that. The developer is unable to update the original app anymore, so it's been unpublished.
The old beta version of the app is now expired, and directs users to update in order to continue using Solid Explorer. The new version, going by the moniker Solid Explorer Beta2, has a few improvements, and the changelog includes a little apology: