How to limit "Forwarding" of a shared Excel Workbook

I think this is a good suggestion which you can make through the Help->Feedback option.  It would be nice to have more share priveledge levels and have an option, as the owner, to be notified if other were shared.  

That said, that is not a confidentiality or true security type of setting as the other user could easily copy and paste or save as or what not and share the information if they want to.  But as the Owner of the document it would be nice to know if it was shared with someone else, who that other person is, and who did the sharing so if you have a depatment of 20 people sharing the book and all of a sudden access was given to someone in another department you 

a) would know about it relatively right away (and not happen to find out 6month later

b) know who did the share so you could remind them not to share with xyz department or people outside the company

c) in general have some added control over your own document

I understand you are looking to restrict sharing of the file. You can try specific people option in the list. That will only allow the people you chose to see the file. others will need access to open the file. When they click on send request you will receive an email to provide access.

Hope this helps.

The legacy security offered by Excel and Outlook is not quite secure enough for situations like this.  If you're clever in Outlook you can modify a message to disable forwarding but nothing is stopping someone from re-creating the email by copy/paste and sending as a new message.

You might look into Sensitivity Labels if your organization uses them.  The "Custom permissions" label would allow you to specifically grant edit/view permissions to people that need to see the workbook.  If someone doesn't have rights then they can't edit the workbook, let alone even open  it.

You may want to check with an Admin to see what labels are available.

Apply sensitivity labels to your files - Microsoft Support