There are many changes within iPadOS15 - of which one is the loss of the familiar “pinned” Today view on the first Home Screen.
With iPadOS15, you are now able to freely position widgets on any of the Home Screens - but you have lost the “compact” first Home Screen layout that shows a compressed 4x6 grid of App/Folder icons andToday view in Landscape screen orientation.
Widgets placed on a Home Screen now individually occupy one or more elements of a 4x6 icon grid. Home Screens without Widgets retain the familiar 5x6 icon grid. The slide-over Today View, previously seen in Portrait screen orientation from the first Home Screen, is still present - and is now also seen in Landscape. However, the revised slide-over does not incorporate the Date and Time panel.
More information about the new Widgets can be found here:
Use widgets on your iPad - Apple Support
The iPad User Guide is also a rich source of information - and is especially useful in becoming familiar with the many changes and new features introduced in iPadOS15. The Guide is available both online using a web browser (such as Safari), or as an Apple Books download:
iPad User Guide - Apple Support
https://books.apple.com/book/id1567104892
Apple do invite constructive feedback, comments and feature requests via their product feedback pages. For iPad and iPadOS:
Feedback - iPad - Apple