The app is shutting down and the company is already notifying users.
Twitter user, @MattNavarra shared the announcement post:
The Direct app is going away
In the coming month, we’ll no longer be supporting the Direct app. Your conversation will automatically move over to Instagram, you don’t need to do anything.
The app was announced back in 2017 and the company aimed to transform how people share private messages and share messages separately and not in the main Instagram app.
Instagram will still give updates and improve Direct in the main app.