What do you mean? It's literally just a 2-axis lookup table with voltage on one axis and SOC on the other.

Except it isn’t, not with modern lithium based batteries.

Now most devices use direct voltage, coloumb counting and battery impedance to keep track, and that’s with known batteries.

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.

It sometimes looks like that in the lab, but in real life it doesn't work. Here are two of the MANY situations that result in incredibly misleading voltage readouts:

My battery is 80% charged, and then I go outside where it is 10 degC colder. Suddenly my battery voltage drops by 200mV and my battery looks 20% charged

My battery is 80% charged and then I start playing a video. The increased load drops the battery voltage and suddenly my battery looks 20% charged.