Hide Featured Image Meta Box on Editor Screen

I tested it myself with the code you use and it only works with Gutenburg disabled. It seems to be related to the new editor.

Assuming you want to use the new editor, and depending on your theme and plugins this code may work for you (it worked from my tests):

add_action( 'after_setup_theme', function(){
    // this removes the feature image panel from all your post types 
    // including 'post'
    remove_theme_support( 'post-thumbnails' );
    // include all post-types that use the featured image panel here
    add_theme_support( 'post-thumbnails', array( 'example-post-type' ) ); 

}, 11 );