Topic refers. Is it advisable to do it? Remember back in the days, batteries are fully discharged in a docking, and only then it is re-charged

with the newer batteries its best not to drop them completely. Recharge at 20% left

Not recommended.
I'd charge before it reach 10%

Only the older NI-CD and NI-MH battery pack need to do so... The newer lithium battery no such need. In fact like the previous post here, it's best not to wait until too low... U can simply google and find out easily...

It is better to charge it when it's not less than 20% left as it will be better on the battery on the long term.

charge it when is 25%~ below that is danger and not good for your battery

Most important
Battery will become weak when it is exposed to heat.

Don't use your phone while it's charging.
Because the heat produced while charging + your phone generate when you are using it will shorten the lifespan of ur phone BATT.

n years back, some stupid manufacturer design the charging system run by internal battery. when u drain the battery 100%, u unable to recharge the phone.

correction been done after they found the mistake made.

What brand?