An alternative solution to using the plugin is to either make a cypress command or a module which you can import into your tests like so:
const getDocument = (selector) => {
  return cy.get(selector).its('0.contentDocument').should('exist');
};
const getBody = (selector) => {
  // Find the body of the iframe and wrap it so that more commands can be chained to it
  // https://on.cypress.io/wrap
  return getDocument(selector).its('body').should('not.be.undefined').then(cy.wrap);
};
const iframe = { getBody, getDocument };
export default iframe;
You can then import this into your test file and chain to the getBody method to check things in the iframe
import iframe from '../support/iframe';
describe('Test iframe', () => {
  it('Can interact with iframe elements with Cypress', () => {
    iframe.getBody('iframe[id=iframe]').find('something in iframe');
  });
});
Also note:
if your iframe is cross-domain you will need to set chromeWebSecurity: false in your cypress.config.js otherwise you will not be able to wrap its contentDocument.body