On the post edit side, when trying to delete a row from a repeater, a nice toolbox ask for deletion confirmation.
A similar thing could be very useful on the field group edit side, I deleted a complex repeater more than once just by clicking Delete by mistake…
You should submit this request as a new support ticket here https://support.advancedcustomfields.com/new-ticket/
If you hit delete by mistake you can always reload the edit page. This will reload the field group as it was the last time you saved it. Save field groups often and you can always recover if you fat finger something.(Yes, I have fat fingers :P)