Draining below 20 % is no problem at all, the lower you cycle the batteries the longer they last. Lower voltage = lower degradation. A really bad, supposedly knowledgeable website originally pushed that and other nonsense with cycling 20 to 80 % or what it was. It is now getting repeated everywhere. You can go down to 2 V (that is below 0 % SOC) and it will only damage the cell slightly faster over the next 100 cycles(!) of always(!) going down to 2 V. Going down to 3 V or 0 % SOC does nothing. Personal experience: All the batteries I ever had accidentally over-discharged recovered just fine, they were well below even 2 V. No increased leakage, no (measurable) capacity loss. Here an actual source instead of just me japping. Look at figure 7. Even down to 1.2 V and the cells still took 50 cycles until they were at 80 % remaining capacity, so around 10x faster degradation at EXTREMELY low voltages. 0 % SOC is often 3 V, for comparison.

I assume that, since the voltage drops very rapidly at the end of discharge, that people actually ran their battery down into the ground, reverse biasing (negative voltage) the most depleted cell(s), and with that were doing the real damage in one single event in seconds. That is a very different thing and we have BMS monitoring individual cell voltages to prevent that.

While writing this the evidence I provide still feels a bit too weak, like someone counters "see lower IS worse" when consumer products, let alone proper battery packs, can not be discharged nearly that far. So here more data with a clear "line go up", so more degradation with higher SOC all the way from 0 to 100 with no anomaly at the lower end and across 3 different cell chemistries (NCA, NMC and LFP).

Yeah but they asked about first gen Nissan Leafs which, yeah, none of those batteries are holding any type of up unless you regularly drive less than 30 miles day.

Wow I didn't know any of this except slow charging. The ac thing is pretty cool.