There are many changes within iPadOS15 - and major changes to Safari and its management of Bookmarks is only one of the significant changes.
You are not the first to make the observation concerning the major limitations of the iPadOS15 Safari Sidebar - and certainly won’t be the last. Here is just one of the many discussions about this topic:
https://discussions.apple.com/thread/253208195?answerId=256003346022
Apple do invite constructive feedback, comments and feature requests via their product feedback pages. For iPad and iPadOS:
Feedback - iPad - Apple
You are encouraged to make use of this facility in expressing any views concerning issues, bugs, or functionality changes of which you are unhappy.