Custom field metabox not showing in back-end

I recently updated my website after a while, and the custom fields meta box is not showing in the editor anymore. It isn’t showing under “Screen Options” either. Any ideas why this could be, and how to get it back?

It turns out the latest Advanced Custom Fields update (from version 5.6.0 on) removes the core custom fields metaboxes by default.

The way to restore it was to add a filter in functions.php:

add_filter('acf/settings/remove_wp_meta_box', '__return_false');