I am currently using the Programming layout. iPhone 7 and iOS 10.1.1
In every "mode" except dec mode (hex,bin,oct) if I hold a button to edit the layout, then choose "Edit" at the bottom to edit that button, the "multiply" (X) button is missing as a selectable button.
So if I were to edit the existing multiply button, then delete it from the layout, there would be no way to get it back again, other than reset all buttons to default.
When I did delete it as a test, it was actually removed from all modes (dec,bin,hex,oct). So I thought that maybe if I simply added it under dec mode, it would add it back under all modes. It did, sort of. It's enabled in dec mode, but disabled now in all other modes (hex,bin,oct). So it's there but just not accessible.