Hello, is it possible to reference outputs of a stack that is stored on S3?

yes

is there any documentation for that?

just keep in mind if you want to use stackrefs with self-managed backends, they only work for stacks that use the same backend/bucket. you can't use it to reference a stack that exists in a different bucket

Thanks, i noticed. I was using different paths on the bucket for each project, but that wasn't an option.

It is an option if you read my second link