For over a month now, Twitter has had a PWA available in the Microsoft Store. Progressive Web Apps are a new feature in the Windows 10 April 2018 Update, which allow a web app to work more like a native app. The app was also pushed yesterday to those that are on Windows 10 version 1709 - including phones - which doesn't support PWAs, breaking things like notifications and Live Tile support.
Since the April 2018 Update rolled out officially yesterday, Microsoft and Twitter announced the new app. Unfortunately, there was some bad news as well, which is that if you're not on a device that gets the PWA - meaning Windows 10 version 1709 or 1803 - your Twitter app will be unsupported as of June 1.
This means that you'll no longer be able to use a first-party Twitter client on the following platforms: Windows Phone 7, Windows Phone 8, Windows Phone 8.1, Windows 8.1, Windows 10 Mobile versions 1703 and earlier, and Windows 10 for PCs version 1703 or earlier. Neowin was told by a Twitter spokesperson that the legacy apps will no longer be offered beginning today, and will stop working on June 1.
The news isn't exactly surprising. Most of the now-deprecated apps haven't received updates in a long time anyway, and still limit users' tweets to 140 characters. Obviously, using a browser or a third-party client was going to provide a better experience anyway.