Intercept 404 and try to serve another content if exists?

In WP, the 404.php template is used if a page is not found. If 404.php is not in your theme, then a generic Not Found is returned.

Make a child theme (since you never want to change theme code directly), then copy the 404.php file from the theme folder into your child theme folder. Then change that child theme 404.php to do what you want.

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