Hide specific attachments from media library.

hide_from_media_library.php

<?php
/**
 * Hide specific media from media library.
 */

/**
 * Case 1. Do not allow gif.
 *
 * This filter will be triggered by media library's AJAX action.
 *
 * @param array $args Passed to WP_Query.
 */
add_filter( 'ajax_query_attachments_args', function( $args ) {
	// Get all mimes from core.
	$all_mimes   = get_allowed_mime_tyes();
	// List not allowed mimes.
	$not_allowed = ['image/gif'];
	$args['post_mime_type'] = $all_mimes;
	return $args;
} );


/**
 * Case 2. Exclusive condition with post meta.
 *
 * Hide attachments which has some extra post meta.
 * meta_query is not available because it trys inner join.
 * Let's suppose meta_key 'img_src' is not in 'twitter' and 'instagram'.
 */

// Add special query var.
add_filter( 'query_vars', function( $vars ) {
	$vars[] = 'special-img';
	return $vars;
} );

// Mark query var on media library.
add_filter( 'ajax_query_attachments_args', function( $args ) {
	$args['special-img'] = true;
	return $args;
} );

// Filter query.
add_filter( 'posts_where', function( $where, $wp_query ) {
	if ( ! $wp_query->get( 'special-img' ) ) {
		return $where;
	}
	global $wpdb;
	$where .= <<<SQL
	    {$wpdb->posts}.ID NOT IN (
			SELECT post_id FROM {$wpdb->postmeta}
			WHERE meta_key   =  'img_src'
			  AND meta_value IN ('twitter', 'instagram')
		)
SQL;
	return $where;
}, 10, 2 );