This script can be executed directly in the browser's console, while visiting Flickr's groups list, to leave all groups

leaveAllFlickrGroups.js

const wait = (ms) => new Promise((resolve) => setTimeout(resolve, ms));

const leaveOneGroup = async (row) => {
  const event = new MouseEvent("click", {
    bubbles: true,
    cancelable: true,
    view: window,
  });

  const moreButton = row.querySelector(".icon-more_horizontal");

  moreButton.dispatchEvent(event);

  await wait(1_000);

  const leaveGroupOption = [
    ...document.querySelectorAll('[role="menuitem"]'),
  ].find((el) => el.textContent.includes("Leave Group"));

  leaveGroupOption.click();

  await wait(1_000);

  const confirmRemoveCheckbox = [...document.querySelectorAll("label")].find(
    (el) => el.textContent.includes("Also remove all the photos")
  );

  confirmRemoveCheckbox.click();

  await wait(1_000);

  const okButton = [...document.querySelectorAll("button")].find((el) =>
    el.textContent.includes("OK")
  );

  okButton.click();
};

const leaveAllGroups = async () => {
  let groupRows = [...document.querySelectorAll("tr[data-group-nsid]")];

  while (groupRows.length > 0) {
    await leaveOneGroup(groupRows[0]);
    await wait(5_000);
    groupRows = [...document.querySelectorAll("tr[data-group-nsid]")];
  }
};