Jim Raykowski, one from LibreOffice developers, realized so much requested feature - Writer Outline folding mode. You should enable Experimental features in Tools ▸ Options ▸ LibreOffice ▸ Advanced dialog to see "Show outline content visibility button" checkbox in Tools ▸ Options ▸ LibreOffice Writer ▸ View dialog.
After checking it you'll can see a button with arrow near any selected heading in your document. Click on it to fold all text from the current heading to next heading. Right click on it to fold all text from current heading to next the same level heading with all its subheadings.
Look at videos created by Jim (I downloaded it from bug report) below for some explanation:
It will be available in next 7.1 release in early February 2021.
Thanks Jim for your great work!