[VygmraMGVl]

This is true for NiCd batteries, not for Li-ion. In general, LiIon degrades at a rate roughly linear to state of charge when not in use.

[Shog9]

IIRC, the danger with storing lithium batteries at a low SoC is that they do have some small amount of self-discharge and can eventually drop to levels that cause severe degradation - and potentially catastrophic failure when next charged.