Cars have had voltage gauges for decades. The really aren’t very useful because essentially by the time the battery reads less than its nominal 12v it’s basically dead.
True but it will at least confirm that the battery is dead, as opposed to something else. I remember back when I was a kid my parents' car would not crank (you hear the solenoid engage but the engine doesn't turn) and nobody had any idea what it was (if you're not a car person and this never happens to you it might not be obvious especially considering all the other electricals still work fine on the slightly lower voltage). A voltmeter showing below 12V or a "low battery - <12.2V" light would've quickly cleared that up.
