Code referenced by a ClassicPress/WordPress theme's functions.php to add RSS 'suffix messages', as used by DanQ.me.

rss-feed-suffixes.php

<?php

/**
 * Returns the DEFAULT "thanks" message for a post. This is determined (functionally-)randomly,
 * using the ID of the post as the random seed.
 */
function q23_rss_thanks_default( $post ) {
  $rss_thanks_messages = [
    "💖 RSS is fantastic, and so are you for using it. 🎆",
    "👏 Congratulations on being an RSS user. 🎉",
    "🥰 You're reading this post via the RSS feed. That makes you one of the best people on the Internet! 🏆",
    "📰 Using a feed reader is the best way to read my blog posts. How clever you are to know that! 🚀",
    "🌟 You're reading this post via the RSS feed, you star! 🌠",
    "🪄 Feeds are wonderful, and you're a wonderful person for using them. 🔮",
    "❤️‍🔥 You're reading this post via the RSS feed. You're on fire! 🔥",
    "🧨 RSS is dynamite! Thanks for subscribing to my blog. 💥",
    "🤘 You're subscribed to DanQ.me using the RSS feed. You rock! 🎸",
    "🕵️ Subscribing to DanQ.me's RSS feeds means that you'll get to see secret bonus posts not publicised on the main site. Clever you! 🧠",
    "🧡 I love RSS feeds. And I love you for using them. 💙",
    "🎗️ Using RSS feeds is a great way to keep up-to-date with my blog. Thanks for subscribing! 🤗",
    "🦸 You're my hero! (For using RSS to follow my blog.) 🥇",
  ];
  $post_id = intval( $post ? $post->ID : 0 );
  return $rss_thanks_messages[ $post_id % count( $rss_thanks_messages ) ];
}

/**
 * Given post content, appends a "thanks for using RSS" message.
 * If the post has a 'rss_thanks_message' custom variable set, use that (this allows a post to
 * override the default message, which I sometimes use in posts specifically ABOUT RSS).
 * Otherwise, uses `q23_rss_thanks_default` to get the default message for this post.
 */
function q23_rss_thanks( $content ) {
  global $post;
  $rss_thanks_message = get_post_meta( $post->ID , 'rss_thanks_message', true );
  $rss_thanks_message = empty( $rss_thanks_message ) ? q23_rss_thanks_default( $post ) : $rss_thanks_message;
  return $content . "<p style=\"margin-top: 0.5ch; padding-top: 0.5ch; border-top: 1px solid #ccc;\">$rss_thanks_message</p>";
}

/**
 * When generating content for RSS feeds, consider adding a thanks message.
 */
add_action( 'the_content_feed', 'q23_rss_thanks' );