EDIT 20/02: anything new? I would like to know the whole story with JP Player Support answers.
Not abnormal for JP server. You can find some similar stories on the Boards.
https://boards.na.leagueoflegends.com/en/c/player-behavior-moderation/A9EHnwuc-received-permanent-ban-for-no-good-reason
https://boards.na.leagueoflegends.com/en/c/player-behavior-moderation/MzshMaVB-jp-server-perma-ban-is-totally-ridiculous?show=flat&page=2
I imagine you just got a 14d ban and you kept being negative. Maybe its "almost" nothing in our western eyes and nothing compared to what people post around here, but its not "nothing" for JP standards.
Working as intended, JP rules are way way stricter than other regions. Needless to say respect and politeness are pillars in the Japanese society and they decided to reflect these values on their server. Dont mind the "extremely inflammatory", its a template in the reform cards.
Remember you didnt get permabanned for one last incident only, you got permabanned on JP server because you kept being negative.