OSD for alternative shortcut has been removed in 9021b3cb, because that approach don't work well with new layout applet.
New approach intends to make OSD behavior almost identical on X11/ Wayland - OSD should only pop up on keyboard shortcut and not on the applet click/mouse wheeling.
Also, it implements OSD for the Main shortcut.
This is a prove of concept version, for proper shortcuts handling we would need to grab only keys we are interested in (that is - producing XK_ISO_Next_Group keysym), as KGlobalAccel does:
https://code.woboq.org/qt5/kf5/kglobalaccel/src/runtime/plugins/xcb/kglobalaccel_x11.cpp.html#125