what you have described as "yellow bar notification" is called Status bar
The Status bar
can't be disabled or enabled using C# or Powershell Because it's not a Server Side property, it's JS SP.UI.Status
class that defined in SP.Js
/ SP.Debug.Js
The alternative solutions to remove/disable the Status bar
:
Using JS
Remove a specific Status Bar
SP.UI.Status.removeStatus(StatusBarID);
Remove all Status Bar
SP.UI.Status.removeAllStatus(true);
Using CSS
#StatusBarID
{
display:none !important
}
Check also How to use Status Bar in SharePoint 2010?