This suggestion is following the new Orion update (Version 0.99.114-beta), and this suggestion.
In that past suggestion, it was notably asked if the side bar could hide the bookmarks and read-later section.
Going from this:
The rational to this suggestion was that the user never reach for bookmarks in this menu when I can go to the menu bar.
I think the tab bar should be more considered as a tool, rather than a way to hide things. By "deconstructing" the sidebar, and having several sides bars living independently from one another, I think the user should have the option to have an unified sidebar, à la Firefox.
Firefox's sidebar allows, in a single click, to have access to Bookmarks, history, synced tabs, and even other extensions (Bitwarden, or side tabs extensions, etc.). I believe user experience would be a lot more swift if we could have something analog to this.
One could argue that everything is accessible through the sidebar quite easily through the keyboard shortcuts (ctrl+cmd+1 and ctrl+cmd+2 for bookmarks and reading list), but there is no keyboard shortcut to switch from any of these sidebars back to the tab sidebar.
I hope this post is clear enough. English isn't my main language and I struggle to find the proper words to argue intelligibly.
Here is a montage of Firefox sidebars for illustration :
All these features are accessible from the same place, by clicking on the small downward arrow near each sidebar title. Which is not possible with the latest Orion version (see screenshot 2).