How to Remove/Close Document Tabs (Side Panel)
If your document has the new side navigation menu (Document Tabs) and you want to delete a specific tab:
On the far-left side of your Google Doc, click on the Show tabs & outlines icon.
Click on the specific tab you want to delete to highlight it.
Click the More options icon (the three vertical dots) next to the tab name.
Note: You can Minimize/Hide the Document Side Panel but you cannot remove the Entire Document Tab or Outline Feature
How to Hide the Document Side Panel
Click on Back Arrow in the Document Side Panel
Or
Click View on top Menu
Select Collapse Tabs & outlines side bar
Quick Keys: Press Ctrl+Alt+H