Even for lithium ion batteries, notoriously hard to track, that's only true for the middle 70-80% of the charging range. It's very easy to check it a lithium ion battery is almost empty, just wait for the voltage to drop below 3.6 or 3.5 and maybe adjust for temperature.
For a CR2032 specifically, you get a nice smooth drop over at least the last third to quarter of its charge. There's no difficulty in detecting that.
Now most devices use direct voltage, coloumb counting and battery impedance to keep track, and that’s with known batteries.