From user:

I noticed this limitation was removed on Coder v2.18.0, but the TF provider v0.0.8 still uses v2.17.x validation rules:
https://github.com/coder/terraform-provider-coderd/blob/v0.0.8/go.mod#L7
https://github.com/coder/terraform-provider-coderd/blob/v0.0.8/internal/codersdkvalidator/group_name.go#L9