Disable Categories for Posts

Removes categories from posts: the Categories menu, the editor panel and the column in the posts list.

Code

add_action( 'init', function () {
    unregister_taxonomy_for_object_type( 'category', 'post' );
}, 99 );

Existing assignments stay in the database; do not use it if your permalinks include %category%.

Install WPIDE on your WordPress site (it is free) and open Code Snippets.
Use snippet above adds it from the library in one click. Or copy the code into a new snippet with the same code type.
It is added inactive: review the code and settings, then switch it on.