2.5V is the lower limit of the cut off voltage of most lithium ion cells, excluding LifePO4 and LTO cells.
It doesn’t really matter if it goes down to 2,5V, but how long it stays below 2,5V, or even worse, 2V.
The max limit for any lithium ion cell is 2V. Below 2V, it becomes critical to charge the cell with a lowest current possible you can use, being 50-100mA.
Otherwise, internal chemical and structural damage can happen very quickly if you charge it too rapidly when below 2V.
This is the reason many chargers now have recovery modes when the cell voltage is too low: if they detect that a cell has too low of a voltage to charge it, they will enter a special low current mode, and when it reaches 3V, it will resume normal charging.