With modern graphite based anode Li-ion batteries 3.5V is about 20% charge remaining
Yes, under no load or very small load.
Below about 15-25% SoC (depending on the cell type), the DC ESR skyrockets, which is why discharge ending voltage is typically set between 2.0 and 3.0V (clearly below the open-circuit voltage of 0% SoC). High-power applications use lower ending voltages. Setting ending voltage to 3.5V only results in 20% SoC with
extremely small loads (like C/20), and if there are any spikes to be supplied by the cell, averaging the voltage sense gets very important. Otherwise, 3.5V is easily seen at 50% SoC under modest load, and it's unreliable, sometimes it's 40%, sometimes 60%, depending on load and temperature. It's OK to base an approximate battery gauge on the cell voltage, but
cutoffs either require being on the steeper part of the curve (typically between 10% and 0% SoC), or using very small currents (end of the CC-CV charging).