Thank you. Added a new item to BREAKING_CHANGES.txt.
149. The TabPanel's HideTabStripItem and UnhideTabStripItem server methods and the hideTabStripItem and unhideTabStripItem JavaScript methods have been removed.
Now each item in a TabPanel has an instance of Ext.tab.Tab which you can show/hide.
JavaScript
App.TabPanel1.items.get(0).tab.hide();
App.TabPanel1.items.get(0).tab.show();
App.TabPanelItem.tab.hide();
App.TabPanelItem.tab.show();
C#
TabPanel1.Call("items.get(0).tab.hide");
TabPanel1.Call("items.get(0).tab.show");
TabPanelItem.Call("tab.hide");
TabPanelItem.Call("tab.show");