Hi,
Thank you for writing to Microsoft Community Forums.
I understand that you want to create some unique shapes by creating gaps on the existing shapes.
We tried reproducing your query on our lab machines and found that currently the feature to edit and make changes to the default shapes is not available. I commend you for bringing this to our notice, as this can be useful for other users as well who wants to create unique shapes.
I would suggest you provide a feedback regarding the same using our feedback hub on Windows 10, so that our engineering team can look into this and try to implement the feature in future build.
Regards,
Prakhar Khare
Microsoft Community – Moderator