$  mkdir -p ~/.terraform.d/plugins/github.com/telmate/proxmox/1.0.0/linux_amd64/
$  ls ~/.terraform.d
checkpoint_cache  checkpoint_signature  plugins
$  ls ~/.terraform.d/plugins
github.com  linux_amd64  terraform-provider-proxmox  terraform-provisioner-proxmox
$  cp -v ~/.terraform.d/plugins/terraform-provi* ~/.terraform.d/plugins/github.com/telmate/proxmox/1.0.0/linux_amd64
'/home/tony/.terraform.d/plugins/terraform-provider-proxmox' -> '/home/tony/.terraform.d/plugins/github.com/telmate/proxmox/1.0.0/linux_amd64/terraform-provider-proxmox'
'/home/tony/.terraform.d/plugins/terraform-provisioner-proxmox' -> '/home/tony/.terraform.d/plugins/github.com/telmate/proxmox/1.0.0/linux_amd64/terraform-provisioner-proxmox'
$  ls -l ~/.terraform.d/plugins/github.com/telmate/proxmox/1.0.0/linux_amd64
total 63M
-rwxr-xr-x 1 tony tony 33M Oct  7 09:33 terraform-provider-proxmox
-rwxr-xr-x 1 tony tony 31M Oct  7 09:33 terraform-provisioner-proxmox

$ cd sysadmin-provisioning/proxmox/terraform/staging
$ cat versions.tf
terraform {
    required_version = ">= 0.13"
    required_providers {
        proxmox = {
            versions = "1.0.0"
            source = "github.com/telmate/proxmox"
        }
    }
}

$ terraform init
Initializing modules...

Initializing the backend...

Initializing provider plugins...
- Finding latest version of github.com/telmate/proxmox...
- Finding latest version of -/proxmox...
- Finding latest version of hashicorp/proxmox...
- Installing github.com/telmate/proxmox v1.0.0...
- Installed github.com/telmate/proxmox v1.0.0 (unauthenticated)

Error: Failed to install provider

Error while installing hashicorp/proxmox: provider registry
registry.terraform.io does not have a provider named
registry.terraform.io/hashicorp/proxmox


Error: Failed to install legacy providers required by state

Found unresolvable legacy provider references in state. It looks like these
refer to in-house providers. You can update the resources in state with the
following command:

    terraform state replace-provider registry.terraform.io/-/proxmox github.com/telmate/proxmox

$ cat versions.tf
terraform {
    required_version = ">= 0.13"
    required_providers {
        proxmox = {
            source = "ondrejsika/proxmox"
            version = "2020.9.21"
            # versions = "1.0.0"
            # source = "github.com/telmate/proxmox"
        }
    }
}
$ terraform init
Initializing modules...

Initializing the backend...

Initializing provider plugins...
- Finding latest version of hashicorp/proxmox...
- Finding ondrejsika/proxmox versions matching "2020.9.21, 2020.9.21, 2020.9.21, 2020.9.21, 2020.9.21, 2020.9.21, 2020.9.21, 2020.9.21, 2020.9.21"...
- Finding latest version of -/proxmox...
- Installing ondrejsika/proxmox v2020.9.21...
- Installed ondrejsika/proxmox v2020.9.21 (self-signed, key ID 410FB99F8F08737F)

Partner and community providers are signed by their developers.
If you'd like to know more about provider signing, you can read about it here:
https://www.terraform.io/docs/plugins/signing.html

Error: Failed to install provider

Error while installing hashicorp/proxmox: provider registry
registry.terraform.io does not have a provider named
registry.terraform.io/hashicorp/proxmox


Error: Failed to install legacy providers required by state

Found unresolvable legacy provider references in state. It looks like these
refer to in-house providers. You can update the resources in state with the
following command:

    terraform state replace-provider registry.terraform.io/-/proxmox registry.terraform.io/ondrejsika/proxmox