Mixi administrator has just replied to my query today. Apparently, there are conditions for communities to be auto-deleted that are 1) founder of a community is the only member for a long period of time since first established, 2) zero community topic has been posted for a long period of time since first established. It seems that the second situation has applied to the previous law school community therefore automatically deleted.
That is why I am writing this to list more topics :)
Below is a part of actual message I have received from the mixi admini. (下記はMixi事業部からのメールの一部です)