I think that's true, but it's probably hard to recreate large parts of the index. I don't think there's any mandatory manifest or something like that where you can reliably identify a repo as something that appears in their registry. Probably some missing metadata too.

A while ago I built an alternative plugin manager for Terraform (https://github.com/paraterraform/para) and explored the idea of distributed indices (where you just host a file in the repo as seen in https://github.com/ashald/terraform-provider-stateful/blob/m...).

So at the end of the day, it's either about the effort to inventory the current registry and find sources that are available, or do this on an ongoing as-needed basis (eg something like para would allow an index to be hosted in a github repo managed with PRs).