Rigify rig comes with a handy script to toggle bone layers/ FK-IK switch from the right panel.
But when the charactor is linked into a new file, this script just got lost.
You need to ‘Append’ a text called ‘rig_ui.py’ from that source file and run it once to get the panel back.
append text from source file.
switch to ‘Text Editor’, load the appended ‘rig_ui.py’ and click ‘Run Script’ once.
Now the panel is back. while the ‘Register’ is ticked, the script will automatically run every time the scene is loaded, so you don’t have to do it manually again (for this scene).