Smartphones should not be deeply discharged - the effects on the battery can be devastating. This is exactly what happened to my old iPhone 3GS.
After a while of lying around, the iPhone 3GS (which until then worked without any problems and should actually serve as a "starter" smartphone) is clearly curved and the display has broken apart in individual layers.
Of course, it's impossible to say whether the 100% was due to deep discharge or some other defect - in any case, it doesn't hurt to keep an eye on your smartphone from time to time and charge it once in a while. After all, apart from the defective iPhone, there hasn't been any other damage: a battery that goes up in flames and is left unattended can cause serious problems. Corresponding videos can be found among others at YouTube.
In any case, the iPhone is so demolished that even repair would be impossible - a case for the recycling bin. At least the materials can still be reused in recycling.