I submited a Ticket. Because My guild Avengers of Teldrassil - Gallywix - Alliance was affected with 100% of my items gone, all tabs nearly full. That is the response:
The IMPORTANT part worries me and will be determinant if I will continue playing WOW.
The guild banks and the problem are not forgotten, I promise.
With the release of TWW, there is just an incredible amount going on and the developers are doing what is technically and humanly possible.
But there is at least a few news in the meantime:
1. The developers were able to find the problem and it should not pass again.
2. All guilds that were affected by the problem were recorded.
3. The developers are now working on it.
https://eu.forums.blizzard.com/en/wow/t/guild-bank-items-gone/528709 here you can also find a post about it with comments in blue. If you click on the Blizz → icon on the right, you can jump from bluepost to bluepost.
IMPORTANT: I need to mention, we can’t promise anything, the developers are doing what they can and the plan and goal is, that everything will be saved, but until that happens, we can’t guarantee that it will be possible.
You would therefore have to be patient.
Unfortunately, the whole thing is very complex and affects a lot of guilds, which contributes to the fact that it takes so long.