We needed to change the attribute containing the theme of a campaign page from "_campaign_page_template" to "theme" because the underscore prevented the field from being used in the gutenberg editor. Currently the backend code still has fallbacks to this old value. If no more live content is depending on this old attribute anymore, we can remove the fallbacks from the code.

• Check if there are any more sites that have a value in the postmeta table for "_campaign_page_template" and no value for "theme". Probably best to also check dev and staging environments, to avoid any confusion or issues during testing/verification.
• If possible update all these campaigns so that they have the "theme" attribute with the value of "_campaign_page_template".
• Maybe get from WPCLI?
• provide recommended solution I(e.g. Script)