leaky-paywall-nag-by-category.php

<?php 

add_filter( 'leaky_paywall_nag_message_text', 'zeen_lp_nag_by_cat', 20, 2 );

function zeen_lp_nag_by_cat( $text, $post_id ) {

    $cats = get_the_category( $post_id );
    
    foreach( $cats as $cat ) {

        if ( $cat->name == 'Blues' ) {
            return 'For all the Blues content, please subscribe.';
        } else if ( $cat->name == 'Jazz' ) {
            return 'For all the Jazz content, please subscribe.';
        }
    }
  
    return $text;
}