Magic Quotes is a PHP function where strings are automatically escaped (special characters are prefixed with a backslash) before being passed. This is considered a bad practice and as of PHP 7.4, they are deprecated.
This article by Sebastian Bergmann explains in detail Why Magic Quotes are gone in PHP 7
So, you can either disable the plugin that is using this unsafe function and use an alternative WordPress plugin.
Or, revert the PHP version to PHP 7.4 or older, where this function is still used.