Display tag admin box like categories, without hierarchy.

It’s easy enough. You just need to remove the default meta box and then add your own one which uses wp_terms_checklist() to output the terms.

I’ve pasted a code example here: https://pastebin.com/MswB57Fn

This works sort of if you make the action hook ‘add_meta_boxes’, but when you select a term and save, it adds a number to the list of taxonomy terms instead of saving properly.

Tags and Categories are handled differently (name vs id AFAIR).

I blogged an example based on this here:
https://www.tcbarrett.com/2012/05/turn-your-taxonomy-into-radio-select-using-wordpress-walker-function/

Thanks TCBarrett. I found a similar article here: https://wp.tutsplus.com/tutorials/creative-coding/how-to-use-radio-buttons-with-taxonomies/ There was some good info in the comments that helped me out.

Here is my final code, to make a non-hierarchical custom taxonomy named ‘mens_rings’ display in the admin like categories would, for the custom post type ‘product’:

add_action( 'add_meta_boxes', 'gpj_change_meta_box');
    function gpj_change_meta_box() {
    remove_meta_box('tagsdiv-mens_rings', 'product', 'normal');
    add_meta_box( 'gpj-mens_rings', 'Men\'s Rings','gpj_mytaxonomy_metabox','product' ,'side','core');
}  

    function gpj_mytaxonomy_metabox($post) {  

        $taxonomy = 'mens_rings';  

        // all terms of ctax
        $all_ctax_terms = get_terms($taxonomy,array('hide_empty' => 0)); 

        // all the terms currenly assigned to the post
        $all_post_terms = get_the_terms( $post->ID,$taxonomy );  

        // name for each input, notice the extra []
        $name = 'tax_input[' . $taxonomy . '][]';  

        // make an array of the ids of all terms attached to the post
        $array_post_term_ids = array();
        if ($all_post_terms) {
            foreach ($all_post_terms as $post_term) {
                $post_term_id = $post_term->term_id;
                $array_post_term_ids[] = $post_term_id;
            }
        }

        ?>

<div id="taxonomy-<?php echo $taxonomy; ?>" class="categorydiv"> 

        <input type="hidden" name="<?php echo $name; ?>" value="0" />

        <ul>
            <?php   foreach($all_ctax_terms as $term){
                    if (in_array($term->term_id, $array_post_term_ids)) {
                        $checked = "checked = ''";
                    }
                    else {
                        $checked = "";
                    }
                $id = $taxonomy.'-'.$term->term_id;
                echo "<li id='$id'>";
                echo "<input type='checkbox' name='{$name}'id='in-$id'"
                . $checked ."value='$term->slug' /><label> $term->name</label><br />";
               echo "</li>";
            }?>
       </ul>
</div>
<?php
    }

Instead of the first ‘add action’ it should be this:

// remove old meta box
add_action( 'admin_menu', 'gpj_remove_meta_box');
function gpj_remove_meta_box() {
   remove_meta_box('tagsdiv-mens_rings', 'product', 'normal');
} 

//Add new taxonomy meta box
 add_action( 'add_meta_boxes', 'gpj_add_meta_box');
 function gpj_add_meta_box() {
     add_meta_box( 'gpj-mens_rings', 'Mens Rings','gpj_mytaxonomy_metabox','product' ,'side','core');
 }