Oh but you have some points when saying that, as reading this thread https://discussions.apple.com/thread/8346176, maybe Mac book have mechanism to auto adjust when battery is fully charged. And the fact that we have to unplug the charging cable is actually a myth ^^

But according to this comment: https://discussions.apple.com/thread/8346176?answerId=33252621022#33252621022 , keeping Mac book plugged forever would increase cycle count to some extent.

Honestly. I don't really know whether my swollen battery issue is directly caused by this or not .