go to https://twitter.com/messages. open console, and paste the code

Delete All DM Conversation Twitter

setInterval(() => {
  for (const d of document.querySelectorAll(
    'div[data-testid="conversation"]'
  )) {
    d.click();
  }
  for (const d of document.querySelectorAll(
    'a[aria-label="Conversation info"]'
  )) {
    d.click();
  }
  for (const d of document.querySelectorAll("span")) {
    if (d.innerHTML === "Leave conversation") {
      d.click();
    }
  }
  for (const d of document.querySelectorAll("span")) {
    if (d.innerHTML === "Leave") {
      d.click();
    }
  }
}, 1000);