How can I include ALL post types in the_post_navigation() links, not just current post type?

By default the the_post_navigation() uses the current post type, whatever it may be. Luckily, that function eventually calls get_adjacent_post() which has a few hooks we can use. The below uses get_{$adjacent}_post_where where $adjacent is either “previous” or “next”:

/**
 * Modify the posts navigation WHERE clause
 * to include our acceptable post types
 * 
 * @param String $where - Generated WHERE SQL Clause
 * 
 * @return String
 */
function wpse375885_postnav_where( $where ) {
    
    global $wpdb, $post;
    
    // Return Early
    if( empty( $post ) ) {
        return $where;
    }
    
    $search = sprintf( "p.post_type="%s"", $post->post_type );
    
    // Return Early - $where already modified
    if( false == strpos( $where, $search ) ) {
        return $where;
    }
    
    // Almost all non-builtin in post types
    $acceptable_post_types = array_merge(
        array( 
            'post',
            'page'
        ),
        get_post_types( array( 
            '_builtin' => false
        ) )
    );
    
    $placeholders   = array_fill( 0, count( $post_types ), '%s' );
    $format         = implode( ',', $placeholders );
    
    $replace = $wpdb->prepare( 
        "p.post_type IN ($format)",
        $post_types
    );
    
    return str_replace( $search, $replace, $where );
    
}
add_filter( 'get_next_post_where',      'wpse375885_postnav_where' );
add_filter( 'get_previous_post_where',  'wpse375885_postnav_where' );

Personally, I would replace $acceptable_post_types with an array of my known post types to prevent future installed plugins from being added to the list.
You could also change '_builtin' => true to truly bring in all post types.

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