Go to the old site at the current image clicking an orange circle at the bottom. This circle turns blue if the image has notes.

SankakuButtonToOldSite.user.js

// ==UserScript==
// @name         Button to old page
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Show a button to go to the old site!
// @author       Baku
// @updateURL    https://gist.github.com/kebien6020/f6489944130a8d891dbebffbe4388b08/raw/SankakuButtonToOldSite.user.js
// @downloadURL  https://gist.github.com/kebien6020/f6489944130a8d891dbebffbe4388b08/raw/SankakuButtonToOldSite.user.js
// @match        https://beta.sankakucomplex.com/*
// @grant        none
// @require      https://unpkg.com/xhook@latest/dist/xhook.min.js
// @run-at       document-start
// ==/UserScript==

//patch history object
(function(history){
    var pushState = history.pushState;
    history.pushState = function(state) {
        const retval = pushState.apply(history, arguments);
        if (typeof history.onpushstate == "function") {
            history.onpushstate({state: state});
        }
        return retval;
    };
})(window.history);

(function(window, history) {
  'use strict';

   const CLASSNAME = 'baku-oldpage-container'

   history.onpushstate = window.onpopstate = window.onload = function () {
       const re = /\/post\/show\/(\d+)/
       const matches = re.exec(window.location.pathname)
       if (matches) {
           const id = matches[1]
           const notesClass = window.postsWithNotes && window.postsWithNotes.indexOf(id) !== -1 ? ' notes' : ''
           const button = `
<style>
  .${CLASSNAME} {
    width: 100%;
    pointer-events: none;
  }
  .${CLASSNAME} a {
    position: fixed;
    display:block;
    bottom: 10px;
    right: 10px;
    width: 30px;
    height: 30px;
    background-color: orange;
    border-radius: 50%;
    z-index: 10000;
    pointer-events: auto;
  }
  .${CLASSNAME} a.notes {
    background-color: #2196F3;
  }
  @media (max-width: 992px) {
    .${CLASSNAME} a {
      left: 50%;
      margin-left: -15px;
    }
  }
</style>
<a
    class="${notesClass}"
    href="//chan.sankakucomplex.com/post/show/${id}"
    target="_blank"
>
</a>`
           let container = document.querySelector(`div.${CLASSNAME}`)
           if (!container) {
               container = document.createElement('div')
               container.classList.add(CLASSNAME)
           }
           console.log('Updated redirect to oldpage with id = ', id)
           container.innerHTML = button
           document.body.append(container)
       }
   }

})(window, window.history);


// Save posts with notes



(function () {
    window.postsWithNotes = []
    //modify 'responseText' of 'example2.txt'
    xhook.after(function(request, response) {
        if(request.url.match(/posts.*$/)) {
            let posts = null
            try {
                posts = JSON.parse(response.text)
            } catch (e) {}

            if (posts !== null && Array.isArray(posts)) {
                for (let i = 0; i < posts.length; ++i) {
                    if (Array.isArray(posts[i].tags)) {
                        if (posts[i].has_notes) {
                            window.postsWithNotes.push(String(posts[i].id))
                        }
                    }
                }
                response.text = JSON.stringify(posts)
            }
        }
    });
})()