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It's an option on the inventory screen when storage is open.
EDIT (after 3 for & 3 against): This poll is about the BUTTON, not the right-click -> store-all option on individual items. Though that option could also go as I've been using the keyboard shortcut ever since I learned about it (STOPATTACK+click).
Soon you can use the original client, if you do not like the addition of the "Store all" option. 
(I've changed my vote to "not remove", since I don't think it makes sense to strip down ManaPlus to Mana level, since we should soon have Mana client playable again anyway)
I voted no, it is actually a very useful option to have, does it cause any problem?
To be honest, I would prefer something like "right click on 'store' button > 'store everything' " instead to keep GUI cleaner and avoid accidents.
IDU this one either. Store all has been useful and I use it often, if you mean the right-click option. I voted to keep the function, as the poll is worded that way, but there is no need for a button, if that's what you meant.
I meant the button, not the right click option. Though the right-click option already has a convenient keyboard shortcut.