Terraform doesn't allow any backends other than the ones built-in. So even if you wanted to provide a "b2" backend, it'd have to be built into to terraform.
Using the S3 backend with B2 should work fine since B2 makes a S3 compatible API available, but its made more difficult because the "Amazon Provider Team" is the one that maintains the "s3" backend for Terraform, and they want to do additional validation that matches AWS's expectations.
Using the S3 backend with B2 should work fine since B2 makes a S3 compatible API available, but its made more difficult because the "Amazon Provider Team" is the one that maintains the "s3" backend for Terraform, and they want to do additional validation that matches AWS's expectations.