Middleware was beta in versions prior to v12.2 and not yet covered by semver.
You can continue to use Middleware in versions prior to v12.2. However, you will need to make changes to upgrade to newer versions.
If you're using Next.js on Vercel, your existing deploys using Middleware will continue to work, and you can continue to deploy your site using Middleware. When you upgrade your site to v12.2 or later, you will need to follow the upgrade guide.