WP Search by ID and fallback to regular search query

search-by-id.php

<?php
/**
 * Search by post ID when integer is available in search terms.
 * Fallback to regular query when integer is absent.
 *
 * @param string $search Search sql query string
 * @param WP_Query $query
 *
 * @return string
 */
function wp_search_by_id_only( $search, $query ) {
	global $wpdb;

	if ( empty( $search ) ) {
		return $search; // skip processing - no search term in query
	}

	$q      = $query->query_vars;
	$ids    = [];

	foreach ( (array) $q['search_terms'] as $term ) {
		$term = absint( $term );
		if ( $term > 0 ) {
			$ids[] = $term;
		}
	}

	if ( count( $ids ) > 0 ) {
		$ids_holder = ltrim( str_repeat( ',%d', count( $ids ) ), ',' );
		$search = $wpdb->prepare( "AND ({$wpdb->posts}.ID IN({$ids_holder}))", $ids );
	}

	return $search;
}
add_filter( 'posts_search', 'wp_search_by_id_only', 500, 2 );