o Start Inkscape and in the opening dialog have it create a new document.
o The panel to the right contains the "Layers and Objects" tab and there's a down arrow to the right the opens the menu of all available tabs.
o Close the "Layers and Objects" tab.
o The whole right pane disappears.
o So closing the last tab is equivalent to closing the panel.
o That panel's dropdown is a very convenient way to access the various tabs but it is now gone and the only way to get it back is to click something from the top menus or click a tool that causes the right frame panel to reopen.
o Would be nice if it stayed open when the last tab was closed.

I'm on MacOS 11.6.8 on an old laptop: Macbook Pro late 2013

FYI, the dock can be collapsed by dragging the divider bar to the right, or clicking the three dots in the divider. 

Not exactly the same, but maybe useful.