At Substack, we believe that direct relationships between readers and writers make for a healthier information ecosystem. That's why our business model relies solely on subscriptions instead of revenue from advertisers. That said, you're welcome to host sponsored content and we don't have any policies in place that prohibit publishers from doing so.
If you're thinking about hosting sponsored content, we recommend that you closely review the Publisher Agreement and our Content Guidelines which prohibit certain practices around advertising and monetizing your publication.