I see what you mean, you basically have a section the sidebar where the “archived” projects lists, not being really archived using the feature but more living in a specific area of the sidebar, say at the bottom?
But perhaps isn’t a simple solution then to make an “Archive” category below which you move all these (in sub-categories and sub-sub categories if needed, representing the hierarchy that they are currently in. i.e. create an Archive category and move the current categories of your “archive area” in)? You can then just use the general collapse/expand ability of that archive to quickly hide and show all the archived projects, keeping things clean.
Another feature that might be helpful for you is the Recent projects feature, which kind of allows you to toggle between the Working area and all projects simply based on what you have recently worked on:
Note that if you click and hold the clock-icon you can have more control over what “recent” means when pressing it:
