Hi, @four. You can embedded ads on your site and still use the Starter plan.
We do require a Pro or higher plan if you plan on billing other people for sites you deploy at Netlify. For example, if you are a web designer and you make a website for a local restaurant.
If it was a friend’s restaurant and you made the website for free you could stay on Starter. However, if you billed the restaurant owner for the site you are hosting on Netlify, then a Pro plan would be required.
Again though, if you are creating our own content and monetizing that content with advertisements, that does not require a Pro plan.
Note, you may find upgrading to Pro is a better deal though if your sites get more than 100 GB of data transfer in a single billing cycle (billing cycle ≈ 1 month) . It wouldn’t be required though unless you are billing someone else for the website.
If there are other questions, please let us know.