Hi @dijana
There is already an optional (manual) reminder which you can use as soon as a reviewer has accepted to do the review. You will see the Send Reminder link in the reviewer’s field and you can remind the reviewer before the review deadline if you want. (you can change the wording of the email in Settings/Workflow/Emails.
Maybe this is not what you meant by ‘optional’? Maybe you meant an automated reminder that is optional to use, i.e. selected or unselected in the reivew/workflow settings? (I agree it would be good to have that reminder btw!)
Best wishes,
Emma