UserScript to erase all the deleted custom objects from a Salesforce org with one click. In Classic UI, go to Setup | Create | Objects | Deleted Objects and the script should be available.

Purge All Deleted Objects.js

// ==UserScript==
// @name         Purge All Deleted Objects
// @description  Erase all deleted custom objects from the Salesforce org
// @include      https://*.salesforce.com/p/setup/custent/DeletedEntitiesList?*
// ==/UserScript==

// UserScripts: https://github.com/quoid/userscripts

// Sometimes, when you're testing Salesforce metadata operations, you end up with a 
// lot of deleted custom objects in your org. There's no API to erase a deleted object, 
// and it's tedious to click through them one by one clicking Erase, then confirm, so I 
// created this UserScript to erase all the deleted custom objects from the org with 
// one click. In Classic UI, go to Setup | Create | Objects | Deleted Objects and the 
// script should be available.

const eraseAll = () => {
  let timer = 0;
  let count = 0;
  let done = 0;
  let linkMap = {};

  // Find all the 'Erase' links
  [].slice.call(document.querySelectorAll('a'), 0).forEach(function(aEl) {
    // if the text of the aEl Node equals the text 'Erase':
    if (aEl.textContent === 'Erase') {
      // Build a map so we can get back to the element from the URL
      linkMap[aEl.href] = aEl;
      // Keep track of number of objects so we can refresh the page at the right time
      count++;
      // Fetch the links asynchronously at regular intervals
      setTimeout(() => {
        console.log(`Fetching ${aEl.href}`);
        // Turn link orange when the request is in flight
        aEl.style.color = 'orange';
        fetch(aEl.href).then(response => {
          done++;
          console.log(`Fetched ${response.url}, status: ${response.status}`)
          // Turn link black if the request succeeded, red if it failed
          linkMap[response.url].style.color = response.ok ? 'black' : 'red';
          if (done == count) {
            // We're all done - reload the page to show the empty list
            location.reload();
          }
        });
      }, timer);
      // One per second is about as fast as you can go
      timer += 1000;
    }
  });
    
  console.log(`${count} objects to delete`);
}

const main = () => {
  // Make an 'Erase All' button
  buttonEl = document.createElement("button");
  buttonEl.appendChild(document.createTextNode("Erase All"));
  buttonEl.addEventListener('click', eraseAll);

  // Put the button under the 'Deleted Custom Objects' header above the list
  [].slice.call(document.querySelectorAll('h3'), 0).forEach(function(h3El) {
    // if the text of the h3El Node contains the text 'Deleted Custom Objects':
    if (h3El.textContent.indexOf('Deleted Custom Objects') > -1) {
      h3El.insertAdjacentElement('afterend', buttonEl); 
    }
  });
}

main()