I don’t think it’s the WordPress 4.8.1 release doing that: but, do you have Advanced Custom Fields Installed? If you do, their release 5.6.0 hides the default WordPress custom fields meta box by default.
remove_wp_meta_boxWe added in a new setting in v5.5.13 allowing the default WP custom
field metabox to be removed. Although simple, this can drastically
speed up the load times of the post edit page! From Advanced Custom
Fields version 5.6, this setting will be set to true by default.
Ref: https://www.advancedcustomfields.com/blog/acf-pro-5-6-0-ui-ux/
To bring back the normal WordPress custom fields meta box you can set that filter back to false by adding the following line into your themes functions.php file:
add_filter('acf/settings/remove_wp_meta_box', '__return_false');
Note that you don’t need to create a function called __return_false for your filter. WordPress already has a core function __return_false, which not surprisingly returns the boolean value false when called.
Ref: https://codex.wordpress.org/Function_Reference/_return_false