If you want to ensure an always up to date version, with all files being removed you might want to clear your _site directory before each build. This needs no further packages then the already built in fs package of node. The function rmdirSync(folderPath, {recursive:true}) can accomplish excactly this.
Your .eleventy.js then looks like this:
const fs = require('fs');
// clear site on initial build
fs.rmdirSync('_site', { recursive: true });
console.log('Cleared _site folder');Note that we do not export a function as you would normally do it. This is, because we want to run the folder deletion only once per build or serve!
That means, that this code does not block our subsequent building pipeline. However, when you run Eleventy with eleventy serve you might end up with unwanted files in the end. So always remember to run eleventy build before publishing your site and you will always get a clean version.