Techs with a central body like a CEO can't be tagged as decentralized, less they is nobody who controls the affairs of the system. However, the anonymity of telegram was misused for illicit purposes beyond scams, from the allegations on him that led to this changed of terms, telegram was used for drug trafficking, and distribution of child sexual abuse images. While it's not questionable whether true or not any platform with a strong privacy like telegram will most likely be a cove for bad actors.

As for scam, he fought it using the notoscam bot that enables members to report scammers and a red tag "scam" will be added to the scammer's account.