I've been searching for the solution to this one for ages and have just discovered it's not a GPO thing but potentially an issue with multiple versions of Office on the same PC and/or maybe even a Microsoft bug. I searched the registry for the AccessVBOM Value and found it present in several Office version branches, in the HKCU and HKLM hives and finally the WOW6432 node of the HKLM hive. When I renamed the one below to "AccessVBOM2", the UI option is no longer greyed out:
HKLM\SOFTWARE\WOW6432Node\Microsoft\Office\16.0\PowerPoint\Security\AccessVBOM
These are the other locations I found (for the PowerPoint 2016 app):
HKLM\SOFTWARE\Microsoft\Office\16.0\PowerPoint\Security\AccessVBOM
HKCU\SOFTWARE\Microsoft\Office\16.0\PowerPoint\Security\AccessVBOM