If you want to hide the terrain tab, instead of deleting it from ribbon.xml, change:
<TabPage name="Terrain"> to <TabPage name="Terrain" visible="false">
This will hide the tab without actually deleting it, causing Studio to crash when it tries to make changes to the nonexistent tab.
The plugins in the ribbon.xml under the Terrain tab are the old terrain plugins. When a place is opened, Studio clears the old terrain plugins from the tab and adds in the new ones. This is why emptying the tab in ribbon.xml doesn’t accomplish much.
Since the smooth terrain plugins aren’t hard-coded into Studio, they require a special kind of syntax. Check the Home page which has buttons for smooth terrain for what you need. The formatting for the name is pluginButtonName.pluginToolbarName.studioTabItsFromName This can actually be used to add user plugins to the ribbon as well.
Also, this is more of a feature request than a bug report since ROBLOX never supported modifying the ribbon xml to begin with.