Theme authors should avoid and not to hardcode custom body_class. Main reason for this is being able to remove the class when needed i.e. child themes.

custom-body-class.php

<?php
// if there's a namespace, add it the body class
    $classes[] = esc_attr( $theme_options['themename_namespace'] );
        
    if ( isset( $post ) ) {
        $classes[] = esc_attr( $post->post_type . '_' . $post->post_name );
    }

    if (is_single() ) {
        foreach((wp_get_post_terms( $post->ID)) as $term) {
            // add category slug to the $classes array
            $classes[] = esc_attr( $term->slug );
        }
        foreach((wp_get_post_categories( $post->ID, array('fields' => 'slugs'))) as $category) {
            // add category slug to the $classes array
            $classes[] = esc_attr( $category );
        }
    }

How should this be used exactly?
There is no mention of body_class();

Is this acceptable ?

function wp_body_classes( $classes ) {
    $classes[] = 'class-name';
      
    return $classes;
}
add_filter( 'body_class','wp_body_classes' );