@email@example.com self-hosting^1 is plumbing^2, does not impact key user functionality, and thus not required for an #IndieWeb site.
You can switch your site from self-hosted to service-hosted & back and it won’t affect your domain, permalinks, content, readers, peer-to-peer comments, or any other IndieWeb user features.
Lastly, “self-hosting” means different things to different people. Some insist it means you have personal physical control of hardware, like a server in your home or garage, some are ok with a physical server in a personal colo cage under lock & key, or a shared colo cage, or a virtual “cloud” server without a physical location.
Per the IndieWeb plurality^3 principle, people can use a self-hosted (under any of those definitions) site or a service-hosted site to publish & interact with each other.