TemplateToaster provides the option to enable and disable the 'Copyright', 'Designed By Text' and 'Designed By Link' section in the Footer Block.
Follow the steps to bring back the deleted Copyright section:
Open your ttr file => open Footer tab => Enable the 'Copyright' checkbox => Modify the text accordingly => Now export your theme.
To bring back the default Copyright text follow the steps:
Enable Copyright option as mentioned above => delete the whole content in appeared textarea => Disable and Enable the Copyright option again.