I help out with the IT Operations duties at work. Coming from an Exchange background, changing an employee's (or former) email forwarding in the Google Workspace Admin console is a little different.

You don't change it under the user's properties. It is configured by adding a mail routing rule. For example, here is the process:

1. Go to the Google Admin console.

2. Click on Apps \> Google Workspace > Gmail.

3. Select Default routing.

4. Click on Configure.

5. Under order number 1, enter the email address you want to use to forward emails.

6. Click the button Add more recipients and click on Add to enter the email address of the user that will receive the forwarded messages.

7. Make sure to select Perform this action on non-recognized and recognized addresses in order number 3.

8. Save the changes.

That's it! Hope this saves someone time looking for this solution.