On a Mac, open the files "Start_CurrentWorkspace.xml" and "Essentials_CurrentWorkspace.xml" in this folder (may differ slightly depending on your version of InDesign):

/Users/[user]/Library/Preferences/Adobe InDesign/Version 17.0/en_GB/Workspaces/

Find this line:

<app-container-control-bar id="64" origin="1 44" size="595 37" is-closed="false"/>

And change it to:

<app-container-control-bar id="64" origin="1 44" size="595 37" is-closed="true"/>

Save the XML files, and restart InDesign.

https://community.adobe.com/t5/indesign-discussions/how-to-hide-application-bar/td-p/10752718?profile.language=fr