Ask Electronics

First off: I am NOT an electrician. From what I know modern LiPos are much less likely to suffer catastrophic failure. These will lose charge over time, especially if plugged into something. Unless there is a voltmeter to ensure operation only with charged lipos, however, this will be very slow. A Lipo running out in such a way might swell and definitely be damaged beyond repain. It should probably still not burst into flames. So in short: I think you should be fine.

LiPo batteries will not last 10 years, even if unused.

They will self-discharge in a matter of a few years (the exact time this takes depends a lot on the quality of the cell btw.). After you reach a critical level of discharge the electrodes inside the battery will start to become irreversibly damaged (typically below 2.7V but this can vary a little depending on exact type).
after this damage becomes severe enough the cell can be considered dangerous because trying to recharge it at this point could result in an explosion. the same is true if a LiPo cell is damaged by some other method, like crushing or extreme temperatures.
if you do not recharge a cell that was damaged from a ultra deep discharge you are relatively safe though. the most probable danger from such a cell would then be leaking electrolyte because the hermetic seal could be damaged from long-term corrosion / environmental damage.

It should be kept charged at ~3.9V/cell (~65% charge), assuming 3.7V nominal voltage. Best storage temperature is 10°C, if memory serves, or maybe it was the best for charging. Self-discharge rate is ~15%/year at that temperature (~30% at room temp), so if you charge to 4.05V (80%) every 2-3 years, it should last long enough. All values depend on the exact cell chemistry. Learned on Battery University long ago.

My phone is 7 years old and the original battery is fine.