"mass exodus" ?
I believe that there are over 100 million posts per day.
"The user base became fragmented..."
Really? I follow any number of people there who speak on topics that I am interested in. This includes people who speak out on women's rights, journalists in different parts of the world that cover news from there, various barristers and others involved in the law, the odd satire account or two and, finally, some accounts that post pictures of cute cats.
"...when council and other official bodies decide to close their twitter accounts"
Perhaps they are getting replies to contentious posts they have made? My local council, local police force and local newspaper all have accounts on X.
The local council one is very mundane though. They post job adverts, details of a new primary school being built, a special service for Holocaust Memorial Day, details of an upcoming by-election etc.