Apple announced that "services offered at TestFlightApp.com will no longer be available after February 26, 2015." The announcement contains a few important hints to help transitioning existing apps from TestFlightApp.com to iTunes Connect.
Most importantly, clarifies Apple, existing apps will not be transferred automatically to TestFlight Beta in iTunesConnect, a task which is left to App developers to carry through. A somewhat easier path is available to transfer all testers associated to a TestFlightApp.com account to iTunes Connect. Indeed, developers can export their testers' contact information into a CSV file that can be later imported in iTunes Connect.
App builds that are currently stored in TestFlightApp.com can still be downloaded in the IPA format. Finally, apps that include the TestFlight SDK will function normally but developers are requested to remove the SDK from future releases.
Apple announced the acquisition of Burstly, owner of TestFlight, in February last year. Immediately thereafter, Apple terminated Android support and discontinued the TestFlight SDK, while allowing developers that already owned a TestFlightApp.com account to continue using TestFlightApp services for iOS apps.
At WWDC 2014, Apple announced the integration of TestFlight within the iOS 8 development tool suite. The new service brought some advantages to developers, such as a significantly higher number of permitted testers, going from 100 to 1,000, and the possibility of inviting new testers by sending them an email, which specifically removed the requirement of adding new testers' devices to the app's provisioning profile. On the other hand, Apple introduced a mandatory review process for beta builds and only allowed to have one active build at a time. Both points raised some concerns in developers. Another major limitation of TestFlight Beta is its availability only on iOS 8, which rules out beta testing on iOS 7 and earlier releases, which still account for about 30% of the user base at the moment of this writing.
Alternative services available for beta testing of iOS apps are HockeyApp, recently bought by Microsoft, and Crashlytics, owned by Twitter.