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 answer to YouTube, a standout but not real application, for videos longer than the allowed base. 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 meant to offer to users a shortcut to browse IGTV content directly. So it seems that nobody or very few used this button.
Most people search for IGTV content through other avenues, such as through feed previews, or from the channel IGTV in Explore, or from creator profiles, even from the standalone app. Very few people click on the IGTV icon in the top right corner of the homepage 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.