Verified by adding custom attribute to Satellite 6 provider:

POST /api/providers/1/custom_attributes?provider_class=provider
{
  "action": "add",
  "name": "ca_name_001",
  "value": "ca_value_001"
}


response:
{
  "error": {
    "kind": "bad_request",
    "message": "ManageIQ::Providers::Foreman::Provider does not support management of custom attributes",
    "klass": "Api::BadRequestError"
  }
}