I've recently found out that Korean translation was dropped out from official distribution some time in the past. However, I couldn't find any line stating such changes from the changelogs.

The latest Korean translation I have indicates CompatibilityLevel=4331.

Was it dropped due to the lack of update? If so, hope I could help with the translation.

Added or dropped translations are never added to the changelog.
It was most likely dropped because of a lack of updates, yes.

If you want to translate start by reading the translation guidelines.

You could create a new thread for the Korean translation.
But remember that the best thing todo when you want to later update the language file is to edit your first post in that thread.
Then re-attach your translation file and update the title of that first post.

So, don't create a new thread for each new update, and don't attach the updated file to a new post each time either. Because all those multiple versions will eventually confuse people.

A good example of how to do things is: Brazilian translation for Messenger Plus! Live - 4.81.358
As you'll see only the first post is updated with the new file each time a new version is released and a new post at the end of the thread is created with simply the text "translation updated" (or something similar) to notify subscribed people.

Remember that each new translation must first be verified by others. Then there are some other rules (for example the amount of people actually using the translation) you must comply with before a language becomes officially bundled in Plus!. Since you're not an official translator and since there hasn't been an up-to-date Korean translation for quite a while it is considered new also...