This % all depends on how the battery management system (BMS) was designed to manage the state of charge (SOC). Some BMS may set 100% SOC to 80% capacity to prolong the cell and pack lifetime. Additionally, 0% SOC may be set as high as 20% capacity.
I think that was the case for GM Bolt years ago, but batteries cost a lot of $$ and you won't see more than 5% reserved for buffer these days -- or 95% usuable capacity.
Tesla also had software lock limiting access to extra battery capacity on the Model S -- 70KWh instead of 75KW, or about (7%?). No point in carrying around extra weight that costs $3,000 to unlock, but it probably helped prolong battery life.