Instagram deleted the IGTV button in the top corner of the app, since no one was using it anyway.
The IGTV button was Instagram's response to YouTube, a separate but not real application for longer videos than allowed. The project started in June 2018 with drumbeats, but apparently did not have the luck that its developers expected.
This is because you can watch IGTV videos without the app. Essentially the button was intended to offer users a shortcut to browse directly to IGTV content. So it seems that no one or a few used this button.
Most people search for IGTV content through other channels, such as preview in the Feed, or from the IGTV channel in Explore, or from the profiles of the creators, even from the standalone application. Very few click on the IGTV icon in the upper right corner of the home screen in the Instagram app.
As reported by Instagram, in their attempt to simplify the platform, they remove this icon based on its non-use and comments from the community.