Mine grayed out and unusable yesterday and I tried installing the non-beta without uninstalling the beta.
And I just found out that the Beta is working again and I even tried uninstalling the non-beta and it did still work.
I have since the day it was leaked. even if they started to disable the app who is not it the beta testers I did not uninstall it since it is still working
I still have Twitter Beta installed on my Lumia 521 with 8.1 preview installed. It will open from the app list, and it works from the pinned tile on my start screen as well. I am not part of the private list of testers. In the store, using the link from previous posts, it says it is not available for my device, but it still has the option to view it in my app list. I don't know why it works for my phone. Any ideas why it might?