I think the idea here is that Apple and other tech developers can't sell new products if they work perfect, if you think about it our iPhones are more powerful then desktop Macs we used in the past to edit heavy tasks like animals, videos etc; ..so my guess is Apple is starting over with safari from the ground up but features under the hood do not seem to benefit your interests but secure Apple`s.
if you think about it Safari is now officially primitive compared to most browsers, but this does not mean Apple would allow other browsers to perform better either....so find a way to make it work
The sidebar is not the only issue, here is the whole list so far:
-the side bar no longer allows you to browse fav bookmarks as mentioned here
-the bookmarks bar is now between the tabs and their pages which is even more confusing, but that's the idea
-maybe its me but I have not figured out how to open safari pages on my other devices via cloud any more
-While i can still see all the safari pages open per my devices in the Catalina version of safari I can't since ios15