help to remove last trailing slash using add_rewrite_rule

Replace all your above code with the one below:

<?php

function master_load_ads_txt_template_include( $template ) {
    $is_load_ads_txt = (bool) get_query_var( 'ads-txt' );

    if ( $is_load_ads_txt ) {
        $template = locate_template( 'template-parts/ads-txt.php' );
    }

    return $template;
}

add_filter( 'template_include', 'master_load_ads_txt_template_include' );

function master_load_ads_txt_rewrite() {
    add_rewrite_rule( 'ads.txt', 'index.php?ads-txt=true', 'top' );

    // The line below doesn't work and it's useless.
    // add_rewrite_rule( '^ads.txt/', 'ads.txt', 'top' );
}

add_action( 'init', 'master_load_ads_txt_rewrite' );

function master_load_ads_txt_query_vars( $query_vars ) {
    $query_vars[] = 'ads-txt';

    return $query_vars;
}

add_filter( 'query_vars', 'master_load_ads_txt_query_vars' );

function redirect_canonical_callback( $redirect_url, $requested_url ) {
    $is_load_ads_txt = (bool) get_query_var( 'ads-txt' );

    if ( $is_load_ads_txt ) {
        return $requested_url;
    }

    return $redirect_url;
}

add_filter( 'redirect_canonical', 'redirect_canonical_callback', 100, 2 );

A few notes:

1. template_include is a filter hook, not an action hook. It’s fixed.
2. As pointed in the comments your rule add_rewrite_rule('^ads.txt/', 'ads.txt', 'top'); is useless. It’s fixed.
3. In this case, redirect_canonical should be used within a filter hook, not an action.
4. After putting the code above in your functions.php file don’t forget to flush your permalinks by visiting Settings > Permalinks.