I like the idea and recently saw Capacities redesign their right sidebar. This means that the one on the right is only used for additional information, such as the table of contents, linked objects, and text stats.
I think the new sidebar in Anytype is great for overall navigation for your space. But when it comes to Table of contents and additional info on a per-object basis, a sidebar on the right would come in handy. I thinks the UX would be better compared to the non persistent relations pop up on the right side, which you can currently open to edit relations.
Maybe this is a freature request for a later stage, but since we get the widget system on left sidebar, maybe it’s not even that complicated to implement it on the left as well. 
Regarding usability of Table of Contents: Currently, a huge amount of chapters, let’s say for a book, is rather a painful experience. I have to scroll down for way too long until i reach the actual content. 
At the same time, when jumping to chapter 7 for example, let’s say i want to jump to chapter 3.2 now. Its not possible to do this instatly because i lost the ToC out of sight. A right sidebar would dramatically improve this.
This is what I mean: