With reasons, please!
In my humble opinion, it was unjustified because a sitting leader of country should be allowed, by virtue of office itself. More like "ex-officio". Personal opinion on his polarizability should not influence decision making. As a sitting USA president, he is privy to a lot of secret information, so it is important to hear him. To de-platform a sitting President is too much power in hands of Twitter.
Hadn't he lost the election and his supporters tried to make him president by force by storming Capitol hill, with 4-5 people getting killed in the ensuing violence. Maybe It would have been a different story if he had won , i guess. He got treated like a normal user, his account was first suspended temporarily but at being reinstated he again made a tweet which seemed to cheer the rioters.. this was the last straw and his account got permanently banned. And It was not done based on personal opinion , the polarisation was quite conspicuous and real and the violent mob at Capitol hill was ample testimony to that.
So ya technically he was still a President but a lame duck one when he was banned. He will still be privy to lot of secret information even when he isn't president so I dont know how this matters.
This is based on my limited understanding and I am quite flexible to change my views based on new inputs. Because i ain't no expert at anything.