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<?php
// all
add_filter( 'use_block_editor_for_post', '__return_false' );

// Specific post types
add_filter( 'use_block_editor_for_post_type', 'disable_block_editor', 10, 2 );
function disable_block_editor( $use_block_editor, $post_type ) {
  if ( $post_type === 'page' ) {
    return false;
  }
  return $use_block_editor;
}