The old separate "mobile interface" — a stripped-down, view-and-edit-only client built on an old version of UiUIKit (Universal iPhone UI Kit) — has been retired.
The current AdminLTE-based theme is responsive and mobile-friendly on its own. There's no separate mobile client to switch to anymore: the same interface you use on desktop adapts to phone and tablet screens, with the full feature set rather than the old client's reduced view/edit-only functionality.