The apps built with App Framework in App Designer will still work and, at least for the near future, you can still edit them with App Designer, you just cannot create new ones. Your best option for App Designer is to use Bootstrap.
BTW -- if you used App Framework outside of App Designer (that is, you built your layout by hand without the assistance of App Designer), you can continue to work without any issues. Just be aware that the App Framework developers are not updating that library, so you won't see any improvements in it unless someone decides to take over that open source project.
