The publishing process for the public registry is very prescriptive, and there is indeed a manifest file involved - https://developer.hashicorp.com/terraform/registry/providers...

That helps, but it reads like the manifest is optional.

Correct, the manifest is optional.

It's... effort consuming, but is doable and more like a 1-time effort.

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).