Most of Koreans aren't good at English. As Korean language is unavailable in whispering mode, they face annoying situation. For example.. When both of Koreans need to talk privately in a same room, they have to communicate using their clumsy English. If they fail to communicate properly, both of them come to the lobby and use transaction system where Korean letters are available.
2nd, When one in the lobby needs to talk to another one who is in a room(and vice versa), one in the lobby has to wait until he/she finishes his/her game and comes to the lobby to communicate using transaction system. How uncomfortable this is?   Even if one is good at English but if another one isn't, same situations would happen.
If you make Korean language usable in whispering mode, it would help them a lot.

I believe this is a side effect of removing it in the lobby.
It's been like 6 years since this change came into play, but I think whisper and lobby chat use the same functions, so blocking one blocked the other. To lift the restriction in whispers would lift it in lobby aswell.

Nowadays I'm certain we could fix it, but I'm unsure it's worth making a priority list at the moment.