The direct message button in the Twitter web app appears normally, but has disappeared for almost all Android users and some iPhone users. For users who have this problem, the way to send a DM is; Touching the messages tab and searching for the account you want to send a message to.
Twitter has focused on the Android app in recent weeks. In addition to the Twitter Blue subscription, which brings a few features to the profiles other than the blue tick, the “For You” tab was introduced, which displays the tweets algorithmically, not chronologically.