> https://github.com/opentffoundation/roadmap/issues/24#issuec...
That's the comment that made the issue clear -- specifically TOS were amended for https://registry.terraform.io to state:
> You may download providers, modules, policy libraries and/or other Services or Content from this website __solely for use with, or in support of, HashiCorp Terraform.__
ie., it looks like the intent is "You can't use OpenTF with registry.terraform.io".
IMO, that feels a little petty. But, I guess if OpenTF is taking a position of "Use us instead of Terraform", then they shouldn't expect to get the usage of Hashicorps infra.