This move of allowing private messages of effectively unlimited length is part of Twitter’s long-running effort to upgrade its messaging service in order to compete with market leaders like Facebook’s WhatsApp and Messenger apps. Twitter officially announced that private messaging remains a core part of its experience, and the service has an advantage over other apps due to amount of already existing amazing content on Twitter, which users can now discuss in private conversations without limitations.
Twitter turned its attention to updating its direct messages a few months ago: the company enabled group discussions with multiple participants, and then announcing the change in DM length. Apparently, the company was trying to win back trust from users before they will devote time to the feature, but it seems to be failing, because the changes were announced to have been enforced on August, 12, but still didn’t work – at least for some users.
Twitter was preparing for this move for a while, because it was important for the company to make sure that all 3rd-party clients were updated before it is turned on. Maybe this is where something went wrong, because Twitter releases updates for iOS every day, but none of them allows longer DMs (yet). Desktop version also limits to 140 characters.