Google released a preview of the next version of Android, Android N and they showcased several cool features that would be bundled in this unnamed release. One of the cool functions that is in Android N is Direct reply notifications which allows you to reply to a notification right within the notification shade without necessarily going to the app.
Android N preview was announced a while ago and the purpose of it is to give time developers to update their apps accordingly before it is officially released to all users. Twitter is of course of one of the major developers and it seems they have been tinkering with the direct reply notifications feature as reported by Android Police.
The publication has been using an alpha version of the Twitter for Android app (v 6.3.0) that allows you to reply to a DM right within the notification shade. When you get a DM from someone, you will see a reply button within the notification and tapping it will allow you to write a quick reply to the message.
Apparently Twitter has not effected this change to mentions which is a rather odd omission since it will benefit greatly from the addition. We have seen other apps that feature the quick reply button within the notifications like the one by WhatsApp which is a small but nifty addition. We can now expect Twitter to add more features from Android N like bundled notifications and multi-window support which could be effected later on in the year.