Thank you for the response. The functions.php is stock from the theme however, this is a network installation so that may make a difference I suppose….
My understanding is PHP 8.0 introduced some changes to error handling and reporting. If the plugin code is relying on keys that are not always guaranteed to be present, it can lead to these warnings. Looking at the code I believe it’s recommended to check that the key ‘address_format’ is set.
$address_format = explode( '_', $wpsl_settings['address_format'] );
with something like this
if (isset($wpsl_settings['address_format'])) { $address_format = explode('_', $wpsl_settings['address_format']);