I am testing the migration from Toolset to Meta Box. The custom field IDs originally started with "wpcf-", which is a Toolset standard. Meta Box stripped that from the ID during the migration. After the migration I deleted all Toolset data and the plugins.
However, Kadence Blocks still shows the deleted wpcf-... fields in their dropdowns to select dynamic data sources. This might be caused by a problem in Kadence, but it does mean that the fields are still stored somewhere.
What happens if I delete a field? Is there a trash folder where it can be deleted permanently?
It looks like Kadence search the custom fields in the database (meta key in the table wp_postmeta) and they are still available in the selection of Kadence block.
Meta Box, Toolset or WordPress itself does not have an option to delete the field ID and value in the database. You can use this plugin to delete the custom fields in the database https://wordpress.org/plugins/bulk-delete/.
Or use the function delete_post_meta(), and refer to this topic https://wordpress.stackexchange.com/questions/246684/remove-post-meta-keys.
I got some feedback from Kadence: "It's intentional that we show custom fields; removing the field in Meta Box doesn't remove the data from your posts, and since you can use many things other than Meta Box (like your own custom code) to create custom fields, we intentionally show all the fields that WordPress shows when we do a query, not just the fields that you've created with Meta Box. This means that if you make a field and add content, then that data is registered with WordPress, and you would need to remove it there (not just the plugin) for it to not show in our options."
I did remove most, if not all, field data from the posts before removing the fields themselves. WordPress is not very good in cleaning things up I guess, and after a while it's inevitable to delete old stuff from the database using plugins like you suggested.
Yes, you need to delete the custom field (meta key and meta value) in the database as the suggestion. Let me know if you have any questions.