To hide the Tools pane in Adobe Reader DC 2019.008.20071 or 2019.008.20074, open the registry and navigate to the following registry key:
Here, create a new String Value called “aDefaultRHPViewModeL”. Set its value to “Collapsed”.
When you start Adobe Reader DC after creating that String Value, you should never get the Tools pane anymore.
I haven’t checked if you could create the same String Value in HKEY_LOCAL_MACHINE.