On the topic of being safe; This topic is nothing new in the ham community where most all portable radios are LI battery powered.
My experience is as follows;
I purchased two LI batteries, one for a motorcycle and another for emergency backup (scanners, ham radio). These batteries came with instructions on what to do if the batteries fell below a certain voltage and would no longer take a charge. (A fully charged lithium-ion battery will have a good year of shelf life. But when they go below that cutoff value, the following is what they recommend.) These batteries have protection built in that saves the battery from dropping to zero because once that happens the battery is toast, non-recoverable, junk. So, at a certain voltage they open circuit... nothing in or out! I do not believe the batteries for the SDS100 have this protection built in so they accomplish it by making chargers that sense the voltage level instead. Getting back to the automotive LI batteries, the manufacturer recommends attaching a source that is of the same voltage as what the battery is rated at and then, with this source still attached for a few seconds, connect the charger. Then disconnect the good source and see if the charger works. (Some LI automotive chargers have this ability built in (you don’t need to connect an alternate source) and there are aftermarket LI chargers that also can do this by themselves for our scanner, ham radio needs.)
I suggested trying to recover your battery as others have here. Placing a “like” fully charged battery in parallel with your discharged battery for a few seconds, and then disconnecting them before you place the discharged one in the charger will not cause you or your batteries any undo harm... but...
It’s important to understand Uniden does not recommend trying to “fool” their charger or charging circuits into recovering a battery that may only suffer from a low voltage level that is below that recognized by the charging circuit but still above zero. Uniden may publish the usual cautions but I don’t believe they specifically caution against this? ...I will stand corrected if anyone wants to point out that Uniden does provide guidance.
And the science is the same for the LI batteries that come with the SDS100... they just don’t have that sophisticated method of recovery should the battery drop below a certain voltage, I suspect because the charger circuit they use is designed on the side of consumer safety. The charger is designed to NOT recognize a battery below that cutoff voltage. And because a bad voltage level could be an indicator of a defective battery, which could get hot, swell up, catch fire and explode, the charger won’t work... until you fool it into thinking there is an adequate partial charge in the battery. That is why people are recommending a voltage be placed in parallel (plus to plus, minus to minus) across the discharged battery for a short period of time. Using another (like) charged battery will suffice for this. When you pair up the two batteries monitor any immediate warmth (hold or touch the battery) emanating from the discharged battery and disconnect immediately if the battery gets warm. That is a sure indicator of a battery that has become defective. IT SHOULD NOT BE USED! Dispose of properly (there are drop off bins at Lowe’s or Home Depot).