/usr/share/X11/xkb/compat/basic is in xkb-data 2.10.1-1ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48  | // Minimal set of symbol interpretations to provide 
// reasonable default behavior (Num lock, shift and 
// caps lock and mode switch) and set up the 
// automatic updating of common keyboard LEDs.
default xkb_compatibility "basic"  {
    virtual_modifiers NumLock,AltGr;
    interpret.repeat= False;
    setMods.clearLocks= True;
    latchMods.clearLocks= True;
    latchMods.latchToLock= True;
    interpret Shift_Lock+AnyOf(Shift+Lock) {
	action= LockMods(modifiers=Shift);
    };
    interpret Any+Lock {
	action= LockMods(modifiers=Lock);
    };
    interpret Num_Lock+Any {
	virtualModifier= NumLock;
	action= LockMods(modifiers=NumLock);
    };
    interpret Mode_switch {
	useModMapMods= level1;
	virtualModifier= AltGr;
	action= SetGroup(group=+1);
    };
    interpret Any + Any {
	action= SetMods(modifiers=modMapMods);
    };
    group 2 = AltGr;
    group 3 = AltGr;
    group 4 = AltGr;
    include "ledcaps"
    include "lednum"
    indicator "Shift Lock" {
	!allowExplicit;
	whichModState= Locked;
	modifiers= Shift;
    };
};
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