is it possible to use the self-hosted pulumi instance with an s3 backend, without a subscription? it’s unclear if self-hosted pulumi also requires a subscription

You can use a self hosted backend (e.g S3 bucket) with pulumi open source which is free

"Self-hosted" is not really meaningful in the free/open-source model. You don't need to host a service at all. You just use a file or cloud storage (e.g., S3) as your backend, and the client does all the work, persisting your stack's state into the backend state files.

you might be looking for “self-managed” https://www.pulumi.com/docs/intro/concepts/state/#backends